Implement dual-path session management in LLMTraceRepository.save():
- UoW mode (explicit session provided): flush only, caller controls commit
- Standalone mode (no session): flush + commit + close for durable persistence
This resolves three data-integrity violations:
1. Premature commit of outer UoW transactions
2. Loss of rollback capability for subsequent failures
3. Mismatch between class docstring and implementation
Also adds:
- Input validation: trace must not be None
- Updated BDD step definitions to pass session explicitly in UoW scenarios
- close() method to _BrokenSession mock for proper cleanup path coverage
- CHANGELOG.md entry for issue #7505
- CONTRIBUTORS.md credit for HAL 9000
ISSUES CLOSED: #7505
HAL9000