Expose actual RabbitMQ dead-letter publish outcomes

[KilianBoute]

Summary

Add an optional DeadLetterObserver callback to RabbitOptions so consumers can log actual DLQ publication outcomes instead of inferring them from handler actions.

• Emit once after the final logical publish result, including reconnect attempts, outside publisher locks and before recovery/settlement.
• Distinguish confirmed success, unconfirmed publication, and failed publication; Confirmed is true only on a successful confirmed transfer.
• Include actual source/DLQ queues, reason, retry attempts and allowlisted traceId/media filename correlation. Never expose message bodies or credentials through the callback.
• Cover runtime dead-lettering, retry exhaustion, forced raw transfers and explicit administrative publication; exclude inspection, replay and discard restoration.
• Preserve existing ack, retry, recovery and publishing behavior. No callback is installed by default; callbacks must return promptly and be concurrency-safe.

Validation

Focused mocked outcome, correlation, entry-point, reconnect, retry-exhaustion, settlement, recovery and restoration tests passed, including race checks and vet. Sequence integration passes race tests and vet against this implementation. Live RabbitMQ integration tests were not run.

Scope

RabbitMQ client-initiated publication only. Broker-side dead-letter policies are not visible to this observer. This API enables a separate hub-pipeline-sequence structured-logging follow-up.

[codecov]

Codecov Report

❌ Patch coverage is 90.16393% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/queue/rabbitmq.go	70.00%	4 Missing and 2 partials ⚠️

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