fix(gateway): finalize session-end memory providers with transcript

[MonkeyLeeT]

Summary

• finalize gateway session-end memory providers with the real transcript on session expiry and /new//reset
• reuse a shared helper to flush built-in memory, then call shutdown_memory_provider(history) before closing stale agents
• add regressions for the expiry watcher and reset path so the provider hook stays covered

Root Cause

The gateway already had a built-in memory flush path, but its real session-boundary paths did not finalize MemoryProvider.on_session_end() correctly. Session expiry called provider shutdown without the transcript, and /new//reset only scheduled the built-in flush before closing the cached agent. That meant memory providers never received the actual session messages they need to persist end-of-session state.

Validation

• source .venv/bin/activate && python -m pytest tests/gateway/test_async_memory_flush.py tests/gateway/test_session_model_reset.py tests/gateway/test_session_boundary_hooks.py tests/gateway/test_resume_command.py tests/gateway/test_gateway_shutdown.py -q

Closes #11205.

[MonkeyLeeT]

Closing this duplicate fix in favor of existing PRs already covering #11205.