Add native dead letter queue support for Kafka transport

[jeremydmiller]

Summary

Closes #1903

• Adds opt-in native dead letter queue (DLQ) support for the Kafka transport, routing failed messages to a configurable Kafka topic (default: wolverine-dead-letter-queue) with exception metadata headers (exception-type, exception-message, exception-stack, failed-at)
• Follows the existing Redis DLQ pattern: KafkaListener implements ISupportDeadLetterQueue, KafkaTopic overrides TryBuildDeadLetterSender(), fluent API via EnableNativeDeadLetterQueue() on listener config and DeadLetterQueueTopicName() on transport config
• Includes integration tests, compliance test suite, and documentation

Test plan

• 5 DLQ integration tests pass (message routing, exception headers, default disabled, default/custom topic names)
• 18 DLQ compliance tests pass (TransportCompliance<BufferedComplianceWithDlqFixture>)
• 18 existing buffered compliance tests pass (no regressions)
• 18 existing inline compliance tests pass (no regressions)
• All other Kafka test fixtures pass

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