fix: Prune invalid builder mappings on load (#9376)

## Summary

- extract resolveNode to reusable util
- remove mid builder pruning
- handle missing widgets with label

## Review Focus

`resolveNode` was simplified for subgraphs by calling getNodeById on
each of the subgraphs instead of searching their inner nodes manually.

## Screenshots (if applicable)

"Widget not visible"
<img width="657" height="822" alt="image"
src="https://github.com/user-attachments/assets/ab7d1e87-3210-4e54-876a-07881974b5c7"
/>
<img width="674" height="375" alt="image"
src="https://github.com/user-attachments/assets/c50ec871-d423-43d6-8e1e-7b1a362f621c"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9376-fix-Prune-invalid-builder-mappings-on-load-3196d73d3650811280c2d459ed0271af)
by [Unito](https://www.unito.io)

src/utils/litegraphUtil.test.ts

@@ -15,15 +15,21 @@ import {
   createNode,
   isAnimatedOutput,
   isVideoOutput,
-  migrateWidgetsValues
+  migrateWidgetsValues,
+  resolveNode
 } from '@/utils/litegraphUtil'
 
-vi.mock('@/lib/litegraph/src/litegraph', () => ({
+vi.mock('@/lib/litegraph/src/litegraph', async (importOriginal) => ({
+  ...(await importOriginal()),
   LiteGraph: {
     createNode: vi.fn()
   }
 }))
 
+vi.mock('@/scripts/app', () => ({
+  app: { rootGraph: null }
+}))
+
 vi.mock('@/platform/updates/common/toastStore', () => ({
   useToastStore: vi.fn(() => ({
     addAlert: vi.fn(),
@@ -384,3 +390,53 @@ describe('compressWidgetInputSlots', () => {
     expect(graph.links).toEqual([])
   })
 })
+
+describe('resolveNode', () => {
+  function mockGraph(
+    nodeList: Partial<LGraphNode>[],
+    subgraphs?: Map<string, LGraph>
+  ) {
+    const nodesById: Record<string, LGraphNode> = {}
+    for (const n of nodeList) {
+      nodesById[String(n.id)] = n as LGraphNode
+    }
+    return {
+      nodes: nodeList as LGraphNode[],
+      getNodeById(id: unknown) {
+        return id != null ? (nodesById[String(id)] ?? null) : null
+      },
+      subgraphs: subgraphs ?? new Map()
+    } as unknown as LGraph
+  }
+
+  it('returns undefined when graph is nullish', () => {
+    expect(resolveNode(1, null)).toBeUndefined()
+    expect(resolveNode(1, undefined)).toBeUndefined()
+  })
+
+  it('finds a node in the main graph', () => {
+    const node = { id: 5 } as LGraphNode
+    const graph = mockGraph([node])
+    expect(resolveNode(5, graph)).toBe(node)
+  })
+
+  it('finds a node in a subgraph', () => {
+    const subNode = { id: 10 } as LGraphNode
+    const subgraph = mockGraph([subNode])
+    const graph = mockGraph([], new Map([['sg-1', subgraph]]))
+    expect(resolveNode(10, graph)).toBe(subNode)
+  })
+
+  it('returns undefined when node is not found anywhere', () => {
+    const graph = mockGraph([{ id: 1 } as LGraphNode])
+    expect(resolveNode(999, graph)).toBeUndefined()
+  })
+
+  it('prefers main graph over subgraph', () => {
+    const mainNode = { id: 1, title: 'main' } as LGraphNode
+    const subNode = { id: 1, title: 'sub' } as LGraphNode
+    const subgraph = mockGraph([subNode])
+    const graph = mockGraph([mainNode], new Map([['sg-1', subgraph]]))
+    expect(resolveNode(1, graph)).toBe(mainNode)
+  })
+})