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