feat: expose terminal consumer failure hook

Allow consumers to react once after all processing retries are exhausted while preserving the base acknowledgement policy.

Ref: IT-1033

Hrynco.RabbitMq/README.md

@@ -25,3 +25,10 @@ ownership of acknowledgements and retries.
 Override `TryValidateMessage(...)` for application-specific permanent validation.
 Returning `false` nacks the delivery without requeue before retry processing begins.
 Keep validation errors free of credentials and sensitive payload values.
+
+Override `HandleMessageRetriesExhaustedAsync(...)` when a consumer must react once to a
+terminal processing failure, for example by publishing a neutral failure result to the
+requesting client. The hook runs after the final handler exception and before the original
+delivery is nacked without requeue. Hook failures are logged and do not replace the
+original failure or acknowledgement policy. Application shutdown cancellation does not
+invoke the hook.