KAFKA-17459 Stablize reassign_partitions_test.py (apache#17250)

This test expects that each partition can receive the record, so using a non-null key helps distribute the records more randomly.

Reviewers: Chia-Ping Tsai <[email protected]>

tests/kafkatest/tests/core/reassign_partitions_test.py

@@ -176,7 +176,11 @@ def test_reassign_partitions(self, bounce_brokers, reassign_from_offset_zero, me
         self.producer = VerifiableProducer(self.test_context, self.num_producers,
                                            self.kafka, self.topic,
                                            throughput=self.producer_throughput,
-                                           enable_idempotence=True)
+                                           enable_idempotence=True,
+                                           # This test aims to verify the reassignment without failure, assuming that all partitions have data.
+                                           # To avoid the reassignment behavior being affected by the `BuiltInPartitioner` (due to the key not being set),
+                                           # we set a key for the message to ensure both even data distribution across all partitions.
+                                           repeating_keys=100)
         self.consumer = ConsoleConsumer(self.test_context, self.num_consumers,
                                         self.kafka, self.topic,
                                         consumer_timeout_ms=60000,