fix(gateway): dispatch memory session-end hooks on expiry

[LeonSGP43]

Summary\n- dispatch MemoryProvider session-end hooks from the live gateway agent before the flush worker tears the session down\n- keep the existing flush-agent behavior intact for builtin memory and skills\n- add a regression test covering cached gateway agents\n\nWhy\n- gateway session expiry/reset currently runs the flush worker but never calls the live memory manager's \ hook\n- memory providers that rely on end-of-session extraction therefore miss their final pass on gateway platforms\n\nFixes #11205\n\nTesting\n- python3 -m pytest -o addopts='' tests/gateway/test_flush_memory_stale_guard.py

[alt-glitch]

Duplicate pair: this and #11410 fix the same issue (#11205). #15132 (merged) partially addressed this with on_session_finalize. Maintainer should pick one or confirm #15132 already covers the gap.

[kshitijk4poor]

Thanks @LeonSGP43 — your instinct was right: session expiry (not idle eviction) is the correct boundary for dispatching MemoryProvider.on_session_end(), and you targeted the flush path deliberately.

Closing in favor of #57378 (merged / merging), for two reasons:

1. The integration point this PR hooks — _flush_memories_for_session — no longer exists on current main. That flush path was refactored away; the gateway now fires on_session_end via _cleanup_agent_resources → shutdown_memory_provider(session_messages). So the diff no longer applies cleanly.

2. Even conceptually, dispatching from the flush/expiry path alone doesn't close the underlying race: if the cached agent was already soft-evicted (by the idle-TTL sweep or the LRU cap) before the session expired, the live agent is gone and the hook is skipped regardless.

#57378 fixes it at the source — it keeps a finalizable agent in cache until the session actually expires (idle sweep), and commits on_session_end via the live agent on the LRU-cap path — so the hook fires for every session across every eviction path.

Your correct framing (expiry is the boundary) is credited in the salvage. Thanks!