test: enhance polling behavior test coverage

Add comprehensive tests for polling mechanism and improve timing-
sensitive restart test for better reliability.

New tests:
- Verify no duplicate job processing
- Enforce concurrency limits during job bursts
- Validate priority-based job ordering
- Ensure continuous polling after queue empties
- Handle jobs added during processing

Improvements:
- Refactor "multiple restarts" test to be less timing-dependent
- Add proper wait conditions and timeouts
- Include EmailJob type for better type safety

All 32 tests now pass reliably.