Regression test for #14981. Verifies that _session_expiry_watcher fires
on_session_finalize for each session swept out of the store, matching
the contract documented for /new, /reset, CLI shutdown, and gateway stop.
Verified the test fails cleanly on pre-fix code (hook call list missing
sess-expired) and passes with the fix applied.
- Fire on_session_finalize and on_session_reset in gateway _handle_reset_command()
- Fire on_session_finalize during gateway stop() for each active agent
- Move CLI test from tests/ root to tests/cli/ (matches recent restructure)
- Add 5 gateway tests covering reset hooks, ordering, shutdown, and error handling
- Place on_session_reset after new session is guaranteed to exist (covers
the get_or_create_session fallback path)
