KIP-799 — Align behaviour for producer callbacks with documented behaviour

Discussion Kafka 3.2 Producer

Fixes an inconsistency in `KafkaProducer` callback behavior where `Callback.onCompletion` passes a non-null placeholder `RecordMetadata` in some error paths but null in others, contrary to the Javadoc contract specifying that exactly one of `metadata` or `exception` is null. The inconsistency was introduced in PR #4188 (KAFKA-6180) and can cause NPEs or incorrect error handling in callback implementations.

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