Flaky CI: App Router ISR revalidation tests time out in integration shard

[NathanDrake2406]

Two App Router ISR invalidation tests timed out in PR #1754 CI, while unrelated checks passed. This looks flaky rather than related to the PR diff, which only touched AST helper utilities.

Failing run:

• PR: refactor(plugins): share small AST helper utilities #1754
• Workflow run: https://github.com/cloudflare/vinext/actions/runs/27016811518
• Failing job: Vitest (integration 2/3) https://github.com/cloudflare/vinext/actions/runs/27016811518/job/79733966017
• Aggregate report: https://github.com/cloudflare/vinext/actions/runs/27016811518/job/79734934898

Timed out tests:

• tests/app-router.test.ts:2785 - revalidateTag invalidates App Router ISR page entries by fetch tag
• tests/app-router.test.ts:2812 - revalidatePath invalidates App Router ISR page entries by path tag

Failure snippet from the merged Vitest report:

Test Files 1 failed | 42 passed (43)
Tests 2 failed | 1538 passed | 2 skipped (1542)
Error: Test timed out in 30000ms.
  at tests/app-router.test.ts:2785:3
Error: Test timed out in 30000ms.
  at tests/app-router.test.ts:2812:3

Useful context:

• Check passed on the same commit, including Knip.
• Unit tests and integration shards 1/3 and 3/3 passed.
• The failing tests both hit /revalidate-tag-test and then call /api/revalidate-tag or /api/revalidate-path, so the likely area is App Router dev server ISR/revalidation request lifecycle under CI timing/load.