[vitest-pool-workers] Add regression tests for onUnhandledError callback

[penalosa]

Fixes #11532.

The onUnhandledError callback was not being invoked with @cloudflare/vitest-pool-workers on versions prior to 0.13.0. This was fixed by the nodejs_process_v2 compatibility flag enforcement added in the Vitest 4 support rewrite (#11632) — workerd's native process.on('unhandledRejection') support is what Vitest's internal listenForErrors() relies on.

This PR adds regression tests to ensure the callback continues to work:

1. Unhandled rejection fails the run by default — verifies that an unhandled Promise.reject() causes the test run to exit non-zero
2. onUnhandledError can suppress specific errors — verifies that returning false from the callback prevents the error from failing the run

Note: errors from workers are serialised plain objects (via Vitest's serializeValue), not Error instances — this is consistent with standard Vitest pools (threads/forks). The test uses .message property access rather than instanceof Error, matching the Vitest docs example.

---

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: regression test only, no user-facing change

---

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Latest commit: 357829a

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[ask-bonk]

I've now thoroughly reviewed the PR. Let me assess the changes:

1. The test file follows established patterns - it imports test from ./helpers, uses seed() and vitestRun() fixtures, and checks exitCode and stderr - consistent with console.test.ts, filtering.test.ts, etc.

2. The inline vitest config doesn't use the vitestConfig() helper - but this is intentional and justified. The tests need specific config shapes (one without onUnhandledError, one with it), and the vitestConfig() helper adds extra boilerplate (DeterministicSequencer, 90s timeout) that isn't needed for a single-test-file scenario. Other tests like console.test.ts also sometimes build configs inline.

3. The test logic is sound - The first test verifies that unhandled rejections cause a non-zero exit by default. The second test verifies the onUnhandledError callback can suppress them by returning false. The await new Promise<void>((resolve) => setTimeout(resolve, 50)) correctly ensures the promise rejection becomes "unhandled" before the test finishes.

4. The await on result.exitCode at line 29 is consistent with the helper's interface (exitCode is a Promise<number>). Line 59 also correctly awaits it.

5. The timeout of 45_000 is reasonable for E2E tests that spawn child processes.

6. No missing assertions - the first test checks both exit code and stderr content; the second test checks exit code is 0 (success).

7. The err?.message optional chaining in the seeded config is appropriate since the comment correctly notes errors from workers are serialized plain objects.

This is a clean, well-structured regression test that follows the existing patterns in the test suite. No logic bugs, no security issues, no backward compatibility concerns.

LGTM

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commit: 357829a

[workers-devprod]

Codeowners approval required for this PR:

• ✅ @cloudflare/wrangler

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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