fix(gateway): use datetime cutoff in suspend_recently_active

[tomqiaozc]

Summary

• suspend_recently_active() compared entry.updated_at (a datetime) to time.time() - max_age_seconds (a float), raising TypeError: '>=' not supported between instances of 'datetime.datetime' and 'float' on every gateway startup.
• Replaces the float cutoff with _now() - timedelta(seconds=max_age_seconds) so the comparison is datetime >= datetime.
• Adds 3 tests: suspends recent session, skips old session, and regression test for no TypeError.

Fixes #7966

Test plan

• test_suspends_recent_session — session updated 30s ago is suspended (within 120s window)
• test_skips_old_session — session updated 1h ago is not suspended
• test_no_type_error — regression: no datetime >= float TypeError
• Full session test suite: 63/63 passed

🤖 Generated with Claude Code

[teknium1]

Thanks for this fix, @tomqiaozc! The datetime vs float TypeError in suspend_recently_active() was already resolved on main before this PR was opened.

Evidence:

• Commit 50d86b3c7 (Apr 11, 2026 — fix(matrix): replace pickle crypto store with SQLite, fix E2EE decryption #7981) included an identical fix as a side-effect, replacing _time.time() - max_age_seconds with _now() - timedelta(seconds=max_age_seconds) in gateway/session.py.
• gateway/session.py line 1075 on current main already reads cutoff = _now() - timedelta(seconds=max_age_seconds).
• Issue gateway startup session suspension compares datetime to float #7966 was closed as COMPLETED on April 13, 2026.
• PR fix(session): use datetime for cutoff in suspend_recently_active() #8002 (another duplicate of the same fix) was also closed for the same reason.

Closing as implemented on main. — automated hermes-sweeper review