test(backend): cover default checker failure paths

[pengyuayn]

Summary

• Adds DefaultChecker coverage for the lock-held path so overlapping runs return null and do not release an unacquired lock.
• Adds coverage for submission failures so thrown batch errors are reported as failed batch results.
• Complements the existing batch-timeout test for [Testing] DefaultChecker has no unit tests for its lock, batch-timeout, and submission failure paths #1204.

Validation

• PASS: npm test -- --runInBand src/__tests__/defaultChecker.test.ts
• NOTE: npm run typecheck is currently blocked by an existing unrelated src/middleware/auth.ts error: string | undefined passed where string/ArrayLike is required.

Closes #1204

[ogazboiz]

nice failure-path coverage: the lock-already-held case (returns null, doesn't release a lock it never took) and the submission-throws case (batch recorded failed, failedSubmissions 1, lock released once via the finally path) are exactly the paths worth testing, and the asserted log strings match the source. tests pass 3/3. one thing:

1. the branch is ~85 commits behind main, so the red ci is stale-base (pre-esm-conversion), not your test. please rebase so it runs against current main (and so we confirm the asserted log strings still match the current defaultChecker): git fetch origin && git rebase origin/main && git push --force-with-lease.

rebase and i'll merge.

if you want to keep contributing, join us on Telegram: https://t.me/+DOylgFv1jyJlNzM0