fix: return undefined for muted node output resolution (#9302)

## Summary

Muted (NEVER mode) subgraph nodes throw "No inner node DTO found" during
prompt serialization because `resolveOutput()` falls through to subgraph
resolution for nodes whose inner DTOs were never registered.

## Changes

- **What**: Add early return in `ExecutableNodeDTO.resolveOutput()` for
`NEVER` mode nodes, matching the existing `BYPASS` mode guard. Add 5
tests covering muted, bypassed, and normal mode resolution.

## Review Focus

The fix is a single-line early return. The key insight is that
`graphToPrompt` in `executionUtil.ts` correctly skips `getInnerNodes()`
for muted/bypassed nodes, so their inner DTOs are never in the map — but
`resolveOutput()` was missing the corresponding guard for `NEVER` mode.

Fixes #8986

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9302-fix-return-undefined-for-muted-node-output-resolution-3156d73d3650811e9697c7281f11cf96)
by [Unito](https://www.unito.io)

src/utils/executableGroupNodeDto.test.ts

@@ -0,0 +1,30 @@
+import { createTestingPinia } from '@pinia/testing'
+import { setActivePinia } from 'pinia'
+import { beforeEach, describe, expect, it } from 'vitest'
+
+import {
+  LGraph,
+  LGraphNode,
+  LGraphEventMode
+} from '@/lib/litegraph/src/litegraph'
+
+import { ExecutableGroupNodeDTO } from './executableGroupNodeDto'
+
+describe('Muted group node output resolution', () => {
+  beforeEach(() => {
+    setActivePinia(createTestingPinia({ stubActions: false }))
+  })
+
+  it('should return undefined for NEVER mode group nodes', () => {
+    const graph = new LGraph()
+    const node = new LGraphNode('Muted Group')
+    node.addOutput('out', 'IMAGE')
+    node.mode = LGraphEventMode.NEVER
+    graph.add(node)
+
+    const dto = new ExecutableGroupNodeDTO(node, [], new Map(), undefined)
+    const resolved = dto.resolveOutput(0, 'IMAGE', new Set())
+
+    expect(resolved).toBeUndefined()
+  })
+})

src/utils/executableGroupNodeDto.ts

@@ -24,6 +24,9 @@ export class ExecutableGroupNodeDTO extends ExecutableNodeDTO {
   }
 
   override resolveOutput(slot: number, type: ISlotType, visited: Set<string>) {
+    // Muted nodes produce no output
+    if (this.mode === LGraphEventMode.NEVER) return
+
     // Temporary duplication: Bypass nodes are bypassed using the first input with matching type
     if (this.mode === LGraphEventMode.BYPASS) {
       const { inputs } = this