server-stream: poll session->is_cancelled() in stream_aware_should_stop
Address review feedback from coder543. The cancel propagation through
rd.stop() relies on the slot eventually processing the cancel task and
posting a result that notifies the recv condvar, remove_waiting_task_ids
does not notify directly. Add a defensive poll on session->is_cancelled()
so the producer-side next() loop exits on its next iteration after
cancel() without waiting for the cancel task to round trip through a slot.