Follow-up hardening on @ehz0ah / @harshitAgr's session-switch work (#28296):
- on_session_switch no longer runs the old-session writer-drain + pending-token
GET + commit POST inline on the caller's command thread. /new, /branch,
/resume, /undo call it synchronously, so a slow drain (up to 10s) or wedged
commit blocked the user-facing command — the same hazard #41945 fixed for
end-of-turn sync. State now rotates synchronously (cheap) and the old-session
commit is offloaded to a daemon finalizer (generalized _finalize_session_async).
- Guard the (_session_id, _turn_count) pair with _session_state_lock: sync_turn
runs on the memory-manager executor thread while the session hooks run on the
command thread, so the snapshot+reset vs increment was a cross-thread race.
- _session_needs_commit checks the committed-session guard BEFORE the
turn_count>0 shortcut, closing a double-commit window when a racing sync_turn
re-increments after commit+reset.
- Add a _shutting_down flag so deferred finalizers stop POSTing against a
torn-down client; track all prefetch threads in a set so invalidate/shutdown
join every one, not just the latest slot.
Tests: regression for the non-blocking switch (asserts the caller returns while
a slow drain is parked off-thread) and the committed-guard ordering; updated the
deferred-commit test to the unified finalizer contract.
c835448908