test: harden subgraph test coverage and remove low-value tests (#9967)

## Summary

Harden subgraph test coverage: remove low-value change-detector tests,
consolidate fixtures, add behavioral coverage, and fix test
infrastructure issues. Includes minor production code corrections
discovered during test hardening.

## Changes

- **What**: Comprehensive subgraph test suite overhaul across 6 phases
  - Removed change-detector tests and redundant assertions
- Consolidated fixture helpers into `subgraphHelpers.ts` /
`subgraphFixtures.ts`
  - Added Pinia initialization and fixture reset to all test files
  - Fixed barrel import violations (circular dependency prevention)
  - Added behavioral coverage for slot connections, events, edge cases
  - Added E2E helper and smoke test for subgraph promotion
  - Exported `SubgraphSlotBase` from litegraph barrel for test access
- **Production code changes** (minor correctness fixes found during
testing):
- `resolveSubgraphInputLink.ts`: iterate forward (first-connected-wins)
to match `_resolveLinkedPromotionBySubgraphInput`
- `promotionSchema.ts`: return `[]` instead of throwing on invalid
`proxyWidgets`; console.warn always (not DEV-only)
  - `LGraph.ts`: disconnect-after-veto ordering fix
  - `litegraph.ts`: barrel export swap for `SubgraphSlotBase`
- **Stats**: 349 tests passing, 0 skipped across 26 test files

## Review Focus

- Tests that merely asserted default property values were deleted
(change detectors)
- Fixture state is now reset via `resetSubgraphFixtureState()` in
`beforeEach`
- All imports use `@/lib/litegraph/src/litegraph` barrel to avoid
circular deps
- Production changes are small and directly motivated by test findings

---------

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: bymyself <cbyrne@comfy.org>

src/core/graph/subgraph/resolveSubgraphInputLink.test.ts

@@ -6,7 +6,8 @@ import { resolveSubgraphInputLink } from '@/core/graph/subgraph/resolveSubgraphI
 import { LGraphNode } from '@/lib/litegraph/src/litegraph'
 import {
   createTestSubgraph,
-  createTestSubgraphNode
+  createTestSubgraphNode,
+  resetSubgraphFixtureState
 } from '@/lib/litegraph/src/subgraph/__fixtures__/subgraphHelpers'
 import type { Subgraph } from '@/lib/litegraph/src/subgraph/Subgraph'
 import type { SubgraphNode } from '@/lib/litegraph/src/subgraph/SubgraphNode'
@@ -61,6 +62,7 @@ function addLinkedInteriorInput(
 
 beforeEach(() => {
   setActivePinia(createTestingPinia({ stubActions: false }))
+  resetSubgraphFixtureState()
   vi.clearAllMocks()
 })
 
@@ -121,6 +123,21 @@ describe('resolveSubgraphInputLink', () => {
     expect(result).toBe('seed_input')
   })
 
+  test('resolves the first connected link when multiple links exist', () => {
+    const { subgraph, subgraphNode } = createSubgraphSetup('prompt')
+    addLinkedInteriorInput(subgraph, 'prompt', 'first_input', 'firstWidget')
+    addLinkedInteriorInput(subgraph, 'prompt', 'second_input', 'secondWidget')
+
+    const result = resolveSubgraphInputLink(
+      subgraphNode,
+      'prompt',
+      ({ targetInput }) => targetInput.name
+    )
+
+    // First connected wins — consistent with SubgraphNode._resolveLinkedPromotionBySubgraphInput
+    expect(result).toBe('first_input')
+  })
+
   test('caches getTargetWidget result within the same callback evaluation', () => {
     const { subgraph, subgraphNode } = createSubgraphSetup('model')
     const linked = addLinkedInteriorInput(