[format-errors] Await async handlePrettyErrorRequest in fetch handler's try/catch

[petebacondarwin]

Fixes #12755.

handlePrettyErrorRequest is an async function returning Promise<Response>, but it was returned without await in the fetch handler's try/catch block. If the promise rejects (e.g. Youch throws asynchronously), the catch block never executes — the error counter doesn't increment, Sentry doesn't capture the exception, and the user sees an unhandled rejection instead of the 500 JSON fallback.

The fix is a one-character change: return handlePrettyErrorRequest(payload) → return await handlePrettyErrorRequest(payload).

This PR also adds the first unit tests for this package (for both handlePrettyErrorRequest and reviveError), along with the necessary vitest configuration.

Reviewer notes:

• The vitest.config.mts includes a resolve.alias for promjs because the installed package has a broken main field (lib/index.js doesn't exist; the actual entry is index.js in the package root). This alias lets Vite resolve it in tests.
• vitest is not explicitly listed as a devDependency of format-errors — it resolves from the workspace root. Worth considering whether to add it explicitly.
• No changeset is included because @cloudflare/format-errors is a deployed Worker — changesets for it are disallowed in CI as they would trigger a deployment.

Suggested review focus:

• Verify the promjs alias approach is acceptable vs. alternative fixes.
• The test for the await fix (handlePrettyErrorRequest test) uses vi.doMock + dynamic import() to get a fresh module with a mocked Youch that rejects — confirm this correctly proves the bug scenario.

Devin PR requested by @petebacondarwin
Link to Devin session: https://app.devin.ai/sessions/3ad7146985384db895a3cb4a235cd1bf

---

• 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: this is an internal bug fix with no user-facing API changes

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