fix: Use per-queue DLX exchange overrides and add regression coverage

[frankvdb7]

fix: Use per-queue DLX exchange overrides

Per-queue RabbitMQ DLX overrides were ignored because the transport and queues shared the same DeadLetterQueue instance. When a per-queue override was applied, later uses of the shared instance (or transport default) overwrote the queue’s exchange name, so the queue was declared with the transport’s default DLX instead of the specified per queue DLX.

• Ensure RabbitMQ queues get independent DLQ instances (cloned) so per-queue overrides aren’t overwritten by transport defaults.
• Keep per-queue x-dead-letter-exchange values intact during queue declaration.
• Add regression tests for default+override DLX scenarios and queue declaration arguments.

Closes #1864