Add consistent dead letter queue exception metadata across all transports

[jeremydmiller]

Summary

• Add StampFailureMetadata() helper to DeadLetterQueueConstants that stamps exception-type, exception-message, exception-stack, and failed-at headers on envelopes before moving to DLQ
• Apply consistent metadata stamping to all transport MoveToErrorsAsync implementations: SQS, GCP Pub/Sub, Azure Service Bus (inline + batched), NATS, Pulsar, RabbitMQ (InteropFriendly), BufferedReceiver, and DurableReceiver
• Add UseEnhancedDeadLettering option for RabbitMQ that publishes failed messages to DLQ with metadata headers instead of using native NACK-based dead lettering
• Standardize NATS dead letter headers from non-standard x-dlq-* to DeadLetterQueueConstants headers (keeping x-dlq-original-subject as supplementary)

Test plan

• Unit tests for StampFailureMetadata helper (4 tests passing)
  • Verifies all four headers are set correctly
  • Verifies pre-existing headers are preserved
  • Handles null stack trace gracefully
  • Overwrites previous failure headers on re-stamp
• Integration tests for each transport with docker-compose infrastructure
• RabbitMQ enhanced dead lettering integration test

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