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.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