test: subgraph integration contracts and expanded Playwright coverage (#10123)

## Summary

Add integration contract tests (unit) and expanded Playwright coverage
for subgraph promotion, hydration, navigation, and lifecycle edge
behaviors.

## Changes

- **What**: 22 unit/integration tests across 9 files covering promotion
store sync, widget view lifecycle, input link resolution, pseudo-widget
cache, navigation viewport restore, and subgraph operations. 13
Playwright E2E tests covering proxyWidgets hydration stability, promoted
source removal cleanup, pseudo-preview unpack/remove, multi-link
representative round-trip, nested promotion retarget, and navigation
state on workflow switch.
- **Helpers**: Added `isPseudoPreviewEntry`, `getPseudoPreviewWidgets`,
`getNonPreviewPromotedWidgets` to promotedWidgets helper. Added
`SubgraphHelper.getNodeCount()`.

## Review Focus

- Test-only PR — no production code changes
- Validates existing subgraph behaviors are covered by regression tests
before further feature work
- Phase 4 (unit/integration contracts) and Phase 5 (Playwright
expansion) of the subgraph test coverage plan

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10123-test-subgraph-integration-contracts-and-expanded-Playwright-coverage-3256d73d365081258023e3a763859e00)
by [Unito](https://www.unito.io)

---------

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

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

@@ -161,4 +161,33 @@ describe('resolveSubgraphInputLink', () => {
     expect(result).toBe('ok')
     expect(getWidgetFromSlot).toHaveBeenCalledTimes(1)
   })
+
+  test('returns first link result with 3+ links connected', () => {
+    const { subgraph, subgraphNode } = createSubgraphSetup('prompt')
+    addLinkedInteriorInput(subgraph, 'prompt', 'first_input', 'firstWidget')
+    addLinkedInteriorInput(subgraph, 'prompt', 'second_input', 'secondWidget')
+    addLinkedInteriorInput(subgraph, 'prompt', 'third_input', 'thirdWidget')
+
+    const result = resolveSubgraphInputLink(
+      subgraphNode,
+      'prompt',
+      ({ targetInput }) => targetInput.name
+    )
+
+    expect(result).toBe('first_input')
+  })
+
+  test('returns undefined when all links fail to resolve', () => {
+    const { subgraph, subgraphNode } = createSubgraphSetup('prompt')
+    addLinkedInteriorInput(subgraph, 'prompt', 'first_input', 'firstWidget')
+    addLinkedInteriorInput(subgraph, 'prompt', 'second_input', 'secondWidget')
+
+    const result = resolveSubgraphInputLink(
+      subgraphNode,
+      'prompt',
+      () => undefined
+    )
+
+    expect(result).toBeUndefined()
+  })
 })