Why is testing against an embedded/in-memory Kafka broker preferred over mocking the `KafkaTemplate` entirely?

Mocking `KafkaTemplate` only proves your code CALLED send() with the right arguments — it says nothing about serialization actually working, whether your message satisfies the topic's schema, or whether the producer config itself is valid. An embedded broker (or Testcontainers Kafka) runs real serialization and a real (if temporary) broker, catching a whole class of bugs mocks can't see.