fix(gateway): compare session suspension timestamps correctly

[Orinks]

Summary

• fix gateway startup session suspension to compare datetime values consistently
• add a regression test covering recent session entries with datetime updated_at values

Verification

• pytest tests/gateway/test_session.py::TestSuspendRecentlyActive::test_suspends_recent_datetime_entries -v
• pytest tests/gateway/test_session.py -q

Issue

• fixes the startup warning: Session suspension on startup failed: '>=' not supported between instances of 'datetime.datetime' and 'float'

[alt-glitch]

Likely duplicate of #8129 — same datetime-vs-float comparison bug in suspend_recently_active(). #8129 is the surviving fix attempt per prior triage.

[alt-glitch]

Likely duplicate of #8129

[teknium1]

Automated hermes-sweeper review: this fix is already present on current main.

Evidence:

• gateway/session.py:1450 now computes cutoff = _now() - timedelta(seconds=max_age_seconds), so the cutoff is a datetime, not a float timestamp.
• gateway/session.py:1457 compares entry.updated_at >= cutoff, meaning both sides of the comparison are datetime values.
• git log -S 'cutoff = _now() - timedelta(seconds=max_age_seconds)' -- gateway/session.py points to 413990c94537e9c9da973bb21a6afcd332400b91 as the commit that introduced the current implementation in this main history.
• The discussion correctly noted this as a duplicate of the same suspend_recently_active() datetime-vs-float bug, but the current HEAD code is what makes this PR redundant now.

Thanks for the fix attempt — the behavioral issue it targeted is already covered on main.