refactor: improve progress monitoring and test reliability
- Add poll_posting() generator for cleaner progress tracking
- Replace datetime arithmetic with signal-based timeout to fix timezone issues
- Refactor cluster pool tests to use real components instead of mocks
- Remove obsolete test helper files
- Fix type annotations for max_wait parameter (float -> int)
This change addresses the datetime arithmetic error when mixing timezone-aware and timezone-naive datetimes, and improves test maintainability by using actual database operations instead of mocks.
Co-Authored-By: Claude noreply@anthropic.com