chore: migrate tests from tests-ui/ to colocate with source files (#7811)

## Summary

Migrates all unit tests from `tests-ui/` to colocate with their source
files in `src/`, improving discoverability and maintainability.

## Changes

- **What**: Relocated all unit tests to be adjacent to the code they
test, following the `<source>.test.ts` naming convention
- **Config**: Updated `vitest.config.ts` to remove `tests-ui` include
pattern and `@tests-ui` alias
- **Docs**: Moved testing documentation to `docs/testing/` with updated
paths and patterns

## Review Focus

- Migration patterns documented in
`temp/plans/migrate-tests-ui-to-src.md`
- Tests use `@/` path aliases instead of relative imports
- Shared fixtures placed in `__fixtures__/` directories

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7811-chore-migrate-tests-from-tests-ui-to-colocate-with-source-files-2da6d73d36508147a4cce85365dee614)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: GitHub Action <action@github.com>

tests-ui/tests/litegraph/core/ToOutputRenderLink.test.ts

@@ -1,96 +0,0 @@
-import { describe, expect, it, vi } from 'vitest'
-
-import { ToOutputRenderLink } from '@/lib/litegraph/src/litegraph'
-import { LinkDirection } from '@/lib/litegraph/src/litegraph'
-
-describe('ToOutputRenderLink', () => {
-  describe('connectToOutput', () => {
-    it('should return early if inputNode is null', () => {
-      // Setup
-      const mockNetwork = {}
-      const mockFromSlot = {}
-      const mockNode = {
-        id: 'test-id',
-        inputs: [mockFromSlot],
-        getInputPos: vi.fn().mockReturnValue([0, 0])
-      }
-
-      const renderLink = new ToOutputRenderLink(
-        mockNetwork as any,
-        mockNode as any,
-        mockFromSlot as any,
-        undefined,
-        LinkDirection.CENTER
-      )
-
-      // Override the node property to simulate null case
-      Object.defineProperty(renderLink, 'node', {
-        value: null
-      })
-
-      const mockTargetNode = {
-        connectSlots: vi.fn()
-      }
-      const mockEvents = {
-        dispatch: vi.fn()
-      }
-
-      // Act
-      renderLink.connectToOutput(
-        mockTargetNode as any,
-        {} as any,
-        mockEvents as any
-      )
-
-      // Assert
-      expect(mockTargetNode.connectSlots).not.toHaveBeenCalled()
-      expect(mockEvents.dispatch).not.toHaveBeenCalled()
-    })
-
-    it('should create connection and dispatch event when inputNode exists', () => {
-      // Setup
-      const mockNetwork = {}
-      const mockFromSlot = {}
-      const mockNode = {
-        id: 'test-id',
-        inputs: [mockFromSlot],
-        getInputPos: vi.fn().mockReturnValue([0, 0])
-      }
-
-      const renderLink = new ToOutputRenderLink(
-        mockNetwork as any,
-        mockNode as any,
-        mockFromSlot as any,
-        undefined,
-        LinkDirection.CENTER
-      )
-
-      const mockNewLink = { id: 'new-link' }
-      const mockTargetNode = {
-        connectSlots: vi.fn().mockReturnValue(mockNewLink)
-      }
-      const mockEvents = {
-        dispatch: vi.fn()
-      }
-
-      // Act
-      renderLink.connectToOutput(
-        mockTargetNode as any,
-        {} as any,
-        mockEvents as any
-      )
-
-      // Assert
-      expect(mockTargetNode.connectSlots).toHaveBeenCalledWith(
-        expect.anything(),
-        mockNode,
-        mockFromSlot,
-        undefined
-      )
-      expect(mockEvents.dispatch).toHaveBeenCalledWith(
-        'link-created',
-        mockNewLink
-      )
-    })
-  })
-})