changefeedccl: fix kafka messagetoolarge test failure

Fixes: #93847

This fixes the following bug in the TestChangefeedKafkaMessageTooLarge
test setup:
1. The feed starts sending messages, randomly triggering a
   MessageTooLarge error causing a retry with a smaller batch size
2. Eventually, while the retrying process is still ongoing, all 2000
   rows are successfully received by the mock kafka sink, causing
   assertPayloads to complete, causing the test to closeFeed and run
   CANCEL on the changefeed.
3. The retrying process gets stuck in sendMessage where it can't send
   the message to the feedCh which has been closed since the changefeed
   is trying to close, but it also can't exit on the mock sink's tg.done
   since that only closes after the feed fully closes, which requires
   the retrying process to end.

Release note: None

pkg/ccl/changefeedccl/testfeed_test.go

@@ -1717,6 +1717,7 @@ func (s *fakeKafkaSink) Dial() error {
 				}
 				select {
 				case s.feedCh <- m:
+				case <-kafka.stopWorkerCh:
 				case <-s.tg.done:
 				}
 				return nil