The sync _session_has_compression_in_flight sat on the message hot path
and blocked the event loop twice: under session_store._lock during
_ensure_loaded_locked (JSON read) and via db.get_compression_lock_holder
(SQLite SELECT). Async-ify the method and offload both sources via
asyncio.to_thread; await the call site in _handle_active_session_busy_message.
With the default busy_input_mode=interrupt, a burst of rapid gateway
messages arriving while context compression is in flight could interrupt
the current turn and start a fresh turn against the pre-rotation parent
session. Because compression is interrupt-immune (#23975), the still-
running compression later rotates the id out from under that new turn,
and if the new turn also grew past the compression threshold it started
its own uncancellable compression on the same stale parent — forking
multiple orphaned one-shot sibling continuations (#56391).
While a state.db compression lock is held for the session, demote
'interrupt' busy-input mode to 'queue' semantics (mirroring the subagent
protection in #30170), so the follow-up message waits for the in-flight
compression + its id rotation to land instead of racing a new turn
against the stale parent. Ack copy explains the compression demotion.
Fixes#56391.
