fix(gateway): add freshness gate to resume_pending sessions

[vanthinh6886]

Fixes #46934

When the gateway restarts after a crash, sessions interrupted mid-turn are marked resume_pending=True. If the auto-recovery turn fails, resume_pending is never cleared. After the freshness window expires, these sessions become zombies — user-initiated messages resume the stale session with old conversation history, bypassing idle reset.

Root Cause

In gateway/session.py, get_or_create_session() returned resume_pending sessions unconditionally without checking if they were still fresh.

Fix

Add a freshness gate using last_resume_marked_at against HERMES_AUTO_CONTINUE_FRESHNESS window (default 3600s). If expired, fall through to auto-reset with reason "resume_pending_expired".

Before

elif entry.resume_pending:
    entry.updated_at = now
    self._save()
    return entry  # Always returns stale sessions

After

elif entry.resume_pending:
    _ref_time = entry.last_resume_marked_at or entry.updated_at
    if _fw > 0 and (now - _ref_time).total_seconds() > _fw:
        reset_reason = "resume_pending_expired"  # Fall through to reset
    else:
        entry.updated_at = now
        self._save()
        return entry

Respects HERMES_AUTO_CONTINUE_FRESHNESS=0 to disable the gate (opt-out for users who want the old behavior).

[teknium1]

Merged via #56264 (commit a1f62f4) — your commit was cherry-picked onto current main with your authorship preserved in git log. Thanks!

Note: the exact bug your PR targeted (the unconditional return entry in the resume_pending branch) was already partially addressed on main by #54442, which added _should_reset() there. But that idle/daily policy keys on updated_at, which is bumped on every message — so a zombie session that keeps receiving messages never trips it. Your freshness gate keys on last_resume_marked_at (set once, never bumped), which correctly catches that case. The salvage kept your mechanism and added a single-source-of-truth refactor of auto_continue_freshness_window() plus 3 tests.