fix: ensure graceful consumer shutdown on SIGTERM

[nsweeting]

Summary

Fixes consumer shutdown behavior to prevent unacked messages from accumulating on quorum queues during SIGTERM (e.g. k8s pod termination). Previously, the shutdown sequence had several issues that could leave AMQP channels and messages in a broken state.

Problems Fixed

• Consumer.Supervisor was typed as :worker with 5s shutdown — the entire consumer subtree (all consumers, workers, executers) had only 5 seconds before being :killed. Now correctly typed as :supervisor with :infinity shutdown.
• No consumption cancellation during shutdown — the broker continued delivering new messages while workers were shutting down. Now calls AMQP.Basic.cancel first.
• Workers stopped sequentially — on an N-core machine with N workers, sequential DynamicSupervisor.stop calls could easily exceed the shutdown budget. Now stops all workers in parallel.
• Executer had no terminate/2 — when shut down externally (supervisor shutdown), the spawned message-processing process was killed with no ack or nack. Unacked messages would sit in limbo on the broker until heartbeat timeout. Now nacks with requeue: true as a safety net.
• No explicit shutdown timeouts — all processes used OTP defaults (5s). Now uses a proper timeout chain: Consumer.Supervisor (:infinity) > Consumer.Server (30s) > Workers/Executers (25s).

Corrected Shutdown Sequence

SIGTERM
  -> Consumer.Supervisor gets :shutdown (timeout: :infinity)
    -> Each Consumer.Server gets :shutdown (timeout: 30s)
      -> terminate/2:
        1. AMQP.Basic.cancel — stop new message delivery
        2. Stop all Workers in parallel (25s timeout)
           -> Each Executer.terminate/2: nack with requeue if not completed
        3. AMQP.Channel.close — clean channel shutdown
  -> Producer.Pool, Topology.Server, Connection.Pool shut down after

Changes

• lib/rabbit/broker/supervisor.ex — Consumer.Supervisor child spec gets type: :supervisor
• lib/rabbit/consumer/supervisor.ex — Consumer.Server child specs get shutdown: 30_000
• lib/rabbit/consumer/server.ex — Add cancel_consumer/1, parallel stop_workers/1
• lib/rabbit/consumer/executer.ex — Add terminate/2 with safety-net nack, shutdown: 25_000, completed state tracking
• mix.exs — Version bump to 0.22.0