fix: ensure super.stop() is called exactly once in CanalRabbitMQProducer (#5578)

Previously super.stop() was called both inside the try block and after
the catch blocks. On success it was invoked twice; on IOException it was
never reached. Move super.stop() to a finally block so it executes
exactly once regardless of whether close() succeeds or fails.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>