fix: compare session suspend cutoff as datetime

[ccqqlo]

Summary

• fix gateway session startup suspension to compare datetime values instead of mixing datetime with float timestamps
• skip entries with missing updated_at values
• add a regression test covering recent vs old session suspension behavior

Testing

• source venv/bin/activate && pytest -q tests/gateway/test_session_suspend_recently_active.py
• source venv/bin/activate && pytest -q tests/tools/test_send_message_tool.py tests/gateway/test_session_suspend_recently_active.py

[alt-glitch]

Multiple prior PRs for the same datetime-vs-float bug in suspend_recently_active: #7995, #10397, #7973, #8336 (all closed). This appears to be the surviving fix attempt.