The polling loop in executeUnstableAgentTask only checked session status
and message stability, never checking if the background task itself had
been interrupted. This caused the tool call to hang until MAX_POLL_TIME_MS
(10 minutes) when a task was interrupted by prompt errors.
Add manager.getTask() check at each poll iteration to break immediately
on terminal statuses (interrupt, error, cancelled), returning a clear
failure message instead of hanging.
