[fix] Prevent incompatible connections to SubgraphInputNode occupied slots (#4984)

## Summary

This PR fixes #4681 by building upon the foundation laid in PR #1182
(litegraph.js). It prevents incompatible type connections when dragging
from a normal node's output to a SubgraphInputNode's occupied slot.

Before:


https://github.com/user-attachments/assets/03def938-dccc-4b2c-b65b-745abf02a13b

After:


https://github.com/user-attachments/assets/7a0a2ed4-9ecd-4147-be56-d643d448d4cb

## Background

PR #1182 implemented:
- `isValidTarget()` method in SubgraphInput/SubgraphOutput classes for
validation
- Visual feedback during drag (40% opacity for invalid targets)
- Validation at the slot level

However, there was a missing piece: while the visual feedback correctly
showed invalid targets, the actual connection would still be made when
dropped.

## Changes

This PR extends PR #1182 by adding the missing connection prevention:

1. **Added `canConnectToSubgraphInput()` method** to render link
classes:
   - `MovingOutputLink`
   - `ToOutputRenderLink`
   - `FloatingRenderLink`
- All methods use the existing `SubgraphInput.isValidTarget()` from PR
#1182

2. **Added validation in `LinkConnector.dropOnIoNode()`**:
   - Checks `canConnectToSubgraphInput()` before allowing the connection
   - Logs a warning when rejecting invalid connections
   - Follows the same pattern as regular node connections

3. **Added `isSubgraphInputValidDrop()` method**:
   - Provides validation for hover states
   - Ensures consistent validation across the UI

src/lib/litegraph/test/canvas/LinkConnectorSubgraphInputValidation.test.ts

@@ -0,0 +1,310 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { LinkConnector } from '@/lib/litegraph/src/canvas/LinkConnector'
+import { MovingOutputLink } from '@/lib/litegraph/src/canvas/MovingOutputLink'
+import { ToOutputRenderLink } from '@/lib/litegraph/src/canvas/ToOutputRenderLink'
+import { LGraphNode, LLink } from '@/lib/litegraph/src/litegraph'
+import { NodeInputSlot } from '@/lib/litegraph/src/node/NodeInputSlot'
+
+import { createTestSubgraph } from '../subgraph/fixtures/subgraphHelpers'
+
+describe('LinkConnector SubgraphInput connection validation', () => {
+  let connector: LinkConnector
+  const mockSetConnectingLinks = vi.fn()
+
+  beforeEach(() => {
+    connector = new LinkConnector(mockSetConnectingLinks)
+    vi.clearAllMocks()
+  })
+
+  describe('MovingOutputLink validation', () => {
+    it('should implement canConnectToSubgraphInput method', () => {
+      const subgraph = createTestSubgraph({
+        inputs: [{ name: 'number_input', type: 'number' }]
+      })
+
+      const sourceNode = new LGraphNode('SourceNode')
+      sourceNode.addOutput('number_out', 'number')
+      subgraph.add(sourceNode)
+
+      const targetNode = new LGraphNode('TargetNode')
+      targetNode.addInput('number_in', 'number')
+      subgraph.add(targetNode)
+
+      const link = new LLink(1, 'number', sourceNode.id, 0, targetNode.id, 0)
+      subgraph._links.set(link.id, link)
+
+      const movingLink = new MovingOutputLink(subgraph, link)
+
+      // Verify the method exists
+      expect(typeof movingLink.canConnectToSubgraphInput).toBe('function')
+    })
+
+    it('should validate type compatibility correctly', () => {
+      const subgraph = createTestSubgraph({
+        inputs: [{ name: 'number_input', type: 'number' }]
+      })
+
+      const sourceNode = new LGraphNode('SourceNode')
+      sourceNode.addOutput('number_out', 'number')
+      sourceNode.addOutput('string_out', 'string')
+      subgraph.add(sourceNode)
+
+      const targetNode = new LGraphNode('TargetNode')
+      targetNode.addInput('number_in', 'number')
+      targetNode.addInput('string_in', 'string')
+      subgraph.add(targetNode)
+
+      // Create valid link (number -> number)
+      const validLink = new LLink(
+        1,
+        'number',
+        sourceNode.id,
+        0,
+        targetNode.id,
+        0
+      )
+      subgraph._links.set(validLink.id, validLink)
+      const validMovingLink = new MovingOutputLink(subgraph, validLink)
+
+      // Create invalid link (string -> number)
+      const invalidLink = new LLink(
+        2,
+        'string',
+        sourceNode.id,
+        1,
+        targetNode.id,
+        1
+      )
+      subgraph._links.set(invalidLink.id, invalidLink)
+      const invalidMovingLink = new MovingOutputLink(subgraph, invalidLink)
+
+      const numberInput = subgraph.inputs[0]
+
+      // Test validation
+      expect(validMovingLink.canConnectToSubgraphInput(numberInput)).toBe(true)
+      expect(invalidMovingLink.canConnectToSubgraphInput(numberInput)).toBe(
+        false
+      )
+    })
+
+    it('should handle wildcard types', () => {
+      const subgraph = createTestSubgraph({
+        inputs: [{ name: 'wildcard_input', type: '*' }]
+      })
+
+      const sourceNode = new LGraphNode('SourceNode')
+      sourceNode.addOutput('number_out', 'number')
+      subgraph.add(sourceNode)
+
+      const targetNode = new LGraphNode('TargetNode')
+      targetNode.addInput('number_in', 'number')
+      subgraph.add(targetNode)
+
+      const link = new LLink(1, 'number', sourceNode.id, 0, targetNode.id, 0)
+      subgraph._links.set(link.id, link)
+      const movingLink = new MovingOutputLink(subgraph, link)
+
+      const wildcardInput = subgraph.inputs[0]
+
+      // Wildcard should accept any type
+      expect(movingLink.canConnectToSubgraphInput(wildcardInput)).toBe(true)
+    })
+  })
+
+  describe('ToOutputRenderLink validation', () => {
+    it('should implement canConnectToSubgraphInput method', () => {
+      // Create a minimal valid setup
+      const subgraph = createTestSubgraph()
+      const node = new LGraphNode('TestNode')
+      node.id = 1
+      node.addInput('test_in', 'number')
+      subgraph.add(node)
+
+      const slot = node.inputs[0] as NodeInputSlot
+      const renderLink = new ToOutputRenderLink(subgraph, node, slot)
+
+      // Verify the method exists
+      expect(typeof renderLink.canConnectToSubgraphInput).toBe('function')
+    })
+  })
+
+  describe('dropOnIoNode validation', () => {
+    it('should prevent invalid connections when dropping on SubgraphInputNode', () => {
+      const subgraph = createTestSubgraph({
+        inputs: [{ name: 'number_input', type: 'number' }]
+      })
+
+      const sourceNode = new LGraphNode('SourceNode')
+      sourceNode.addOutput('string_out', 'string')
+      subgraph.add(sourceNode)
+
+      const targetNode = new LGraphNode('TargetNode')
+      targetNode.addInput('string_in', 'string')
+      subgraph.add(targetNode)
+
+      // Create an invalid link (string output -> string input, but subgraph expects number)
+      const link = new LLink(1, 'string', sourceNode.id, 0, targetNode.id, 0)
+      subgraph._links.set(link.id, link)
+      const movingLink = new MovingOutputLink(subgraph, link)
+
+      // Mock console.warn to verify it's called
+      const consoleWarnSpy = vi
+        .spyOn(console, 'warn')
+        .mockImplementation(() => {})
+
+      // Add the link to the connector
+      connector.renderLinks.push(movingLink)
+      connector.state.connectingTo = 'output'
+
+      // Create mock event
+      const mockEvent = {
+        canvasX: 100,
+        canvasY: 100
+      } as any
+
+      // Mock the getSlotInPosition to return the subgraph input
+      const mockGetSlotInPosition = vi.fn().mockReturnValue(subgraph.inputs[0])
+      subgraph.inputNode.getSlotInPosition = mockGetSlotInPosition
+
+      // Spy on connectToSubgraphInput to ensure it's NOT called
+      const connectSpy = vi.spyOn(movingLink, 'connectToSubgraphInput')
+
+      // Drop on the SubgraphInputNode
+      connector.dropOnIoNode(subgraph.inputNode, mockEvent)
+
+      // Verify that the invalid connection was skipped
+      expect(consoleWarnSpy).toHaveBeenCalledWith(
+        'Invalid connection type',
+        'string',
+        '->',
+        'number'
+      )
+      expect(connectSpy).not.toHaveBeenCalled()
+
+      consoleWarnSpy.mockRestore()
+    })
+
+    it('should allow valid connections when dropping on SubgraphInputNode', () => {
+      const subgraph = createTestSubgraph({
+        inputs: [{ name: 'number_input', type: 'number' }]
+      })
+
+      const sourceNode = new LGraphNode('SourceNode')
+      sourceNode.addOutput('number_out', 'number')
+      subgraph.add(sourceNode)
+
+      const targetNode = new LGraphNode('TargetNode')
+      targetNode.addInput('number_in', 'number')
+      subgraph.add(targetNode)
+
+      // Create a valid link (number -> number)
+      const link = new LLink(1, 'number', sourceNode.id, 0, targetNode.id, 0)
+      subgraph._links.set(link.id, link)
+      const movingLink = new MovingOutputLink(subgraph, link)
+
+      // Add the link to the connector
+      connector.renderLinks.push(movingLink)
+      connector.state.connectingTo = 'output'
+
+      // Create mock event
+      const mockEvent = {
+        canvasX: 100,
+        canvasY: 100
+      } as any
+
+      // Mock the getSlotInPosition to return the subgraph input
+      const mockGetSlotInPosition = vi.fn().mockReturnValue(subgraph.inputs[0])
+      subgraph.inputNode.getSlotInPosition = mockGetSlotInPosition
+
+      // Spy on connectToSubgraphInput to ensure it IS called
+      const connectSpy = vi.spyOn(movingLink, 'connectToSubgraphInput')
+
+      // Drop on the SubgraphInputNode
+      connector.dropOnIoNode(subgraph.inputNode, mockEvent)
+
+      // Verify that the valid connection was made
+      expect(connectSpy).toHaveBeenCalledWith(
+        subgraph.inputs[0],
+        connector.events
+      )
+    })
+  })
+
+  describe('isSubgraphInputValidDrop', () => {
+    it('should check if render links can connect to SubgraphInput', () => {
+      const subgraph = createTestSubgraph({
+        inputs: [{ name: 'number_input', type: 'number' }]
+      })
+
+      const sourceNode = new LGraphNode('SourceNode')
+      sourceNode.addOutput('number_out', 'number')
+      sourceNode.addOutput('string_out', 'string')
+      subgraph.add(sourceNode)
+
+      const targetNode = new LGraphNode('TargetNode')
+      targetNode.addInput('number_in', 'number')
+      targetNode.addInput('string_in', 'string')
+      subgraph.add(targetNode)
+
+      // Create valid and invalid links
+      const validLink = new LLink(
+        1,
+        'number',
+        sourceNode.id,
+        0,
+        targetNode.id,
+        0
+      )
+      const invalidLink = new LLink(
+        2,
+        'string',
+        sourceNode.id,
+        1,
+        targetNode.id,
+        1
+      )
+      subgraph._links.set(validLink.id, validLink)
+      subgraph._links.set(invalidLink.id, invalidLink)
+
+      const validMovingLink = new MovingOutputLink(subgraph, validLink)
+      const invalidMovingLink = new MovingOutputLink(subgraph, invalidLink)
+
+      const subgraphInput = subgraph.inputs[0]
+
+      // Test with only invalid link
+      connector.renderLinks.length = 0
+      connector.renderLinks.push(invalidMovingLink)
+      expect(connector.isSubgraphInputValidDrop(subgraphInput)).toBe(false)
+
+      // Test with valid link
+      connector.renderLinks.length = 0
+      connector.renderLinks.push(validMovingLink)
+      expect(connector.isSubgraphInputValidDrop(subgraphInput)).toBe(true)
+
+      // Test with mixed links
+      connector.renderLinks.length = 0
+      connector.renderLinks.push(invalidMovingLink, validMovingLink)
+      expect(connector.isSubgraphInputValidDrop(subgraphInput)).toBe(true)
+    })
+
+    it('should handle render links without canConnectToSubgraphInput method', () => {
+      const subgraph = createTestSubgraph({
+        inputs: [{ name: 'number_input', type: 'number' }]
+      })
+
+      // Create a mock render link without the method
+      const mockLink = {
+        fromSlot: { type: 'number' }
+        // No canConnectToSubgraphInput method
+      } as any
+
+      connector.renderLinks.push(mockLink)
+
+      const subgraphInput = subgraph.inputs[0]
+
+      // Should return false as the link doesn't have the method
+      expect(connector.isSubgraphInputValidDrop(subgraphInput)).toBe(false)
+    })
+  })
+})