When a completed session is no longer returned by session.status(),
allStatuses[sessionID] is undefined. Previously this fell through to
a 'still running' log, leaving the task stuck as running forever.
Match the sync-session-poller pattern: only continue (skip completion
check) when sessionStatus EXISTS and is not idle. When undefined,
fall through to validateSessionHasOutput + checkSessionTodos +
tryCompleteTask, same as idle.
