Configure a RabbitMQ alternate exchange to capture unroutable messages instead of silently dropping them

Verified steps

1. Create a dedicated exchange (commonly a fanout) that will act as the alternate exchange (AE), and bind a queue to it to collect unroutable messages
2. Configure the alternate-exchange argument on your primary exchange, either at exchange.declare time (arguments table key 'alternate-exchange' with the AE's name as value) or via a policy targeting the exchange
3. When arguments and policy both specify an AE for the same exchange, the argument value takes precedence over the policy value
4. Publish a test message with a routing key that matches no bindings on the primary exchange and confirm it is delegated to the AE and lands in the collector queue
5. If a message is routed via the AE, it still counts as 'routed' for the purposes of the publisher's mandatory flag, so publisher-confirms-based unroutable detection alone won't catch it
6. Chain AEs across multiple exchanges if you want a shared, centralized unroutable-message sink for several primary exchanges