Before this, _process_message_background's finally did an unconditional
'del self._active_sessions[session_key]' — even if a /stop/ /new
command had already swapped in its own command_guard via
_dispatch_active_session_command and cancelled us. The old task's
unwind would clobber the newer guard, opening a race for follow-ups.
Replace with _release_session_guard(session_key, guard=interrupt_event)
so the delete only fires when the guard we captured is still the one
installed. The sibling _session_tasks pop already had equivalent
ownership matching via asyncio.current_task() identity; this closes the
asymmetry.
Adds two direct regressions in test_session_split_brain_11016:
- stale guard reference must not clobber a newer guard by identity
- guard=None default still releases unconditionally (for callers that
don't have a captured guard to match against)
Refs #11016
5651a73331