fix(gateway): invalidate agent cache on cross-process session writes (#45966)

[kyssta-exe]

Fixes #45966. When another process (e.g. hermes dashboard) writes to the same session DB, the cached agent's in-memory transcript becomes stale. This fix stores the session's message_count alongside the cached agent and checks it on reuse. If the count has changed (indicating a cross-process write), the cache entry is discarded and a fresh agent is built from the updated DB transcript.

[liuhao1024]

Verification: clean review — agent cache invalidation for cross-process writes

Reviewed the full diff. Implementation is sound:

1. Staleness detection via message_count — queries _session_db.get_session() before acquiring _agent_cache_lock, avoiding holding the lock during I/O. Correct ordering.
2. Defensive None handling — _current_msg_count can be None (session not found, exception); the if _cached_mc is not None and _current_msg_count is not None guard correctly skips invalidation in that case rather than false-positive evicting.
3. Cache tuple migration — (agent, _sig) → (agent, _sig, _current_msg_count) with len(cached) > 2 backward compat for entries cached before this change. No crash on mixed old/new tuples.
4. Resource cleanup on eviction — _cleanup_agent_resources is called on the evicted agent before creating a fresh one, preventing resource leaks from stale agents.
5. Logging — logger.info with both old and new message_count values gives operators clear visibility into when cross-process writes trigger cache misses.

Checked for: race conditions between the pre-lock DB query and the post-lock cache check (benign — worst case is one unnecessary cache miss), dead variables, callback chain issues. No findings.

Tests cover the happy path (cache hit/miss) and the cross-process invalidation scenario. Clean PR.

[alt-glitch]

Duplicate of #45949 — same fix: snapshot the session's on-disk message_count alongside the cached agent and evict+rebuild on cross-process transcript growth. #45949 is the earlier open PR for #45966.

[kyssta-exe]

This PR's fix for #45966 is already merged via #46237. Closing as superseded.