Expire agent.task.status queues of stopped A2A callers (#654)

[rockfordlhotka]

Summary

• AgentHost topics can carry SubscriptionOptions. TopicSubscription gets an optional Options parameter, set through the new AgentHostBuilder.SubscribeTo(topic, SubscriptionOptions) overload. AgentHost.StartAsync now subscribes through the options overload of IMessageSubscriber.SubscribeAsync. Topics registered without options get the same defaults as before, so RabbitMQ declares the same arguments for them. The topic's DispatchConcurrency always wins.
• The A2A caller's agent.task.status subscription (rockbot.{Agent}.agent-task-status) now uses StatusSubscriptionOptions: MessageTtl = 10 min, IdleExpiry = 24 h, DeadLetter = false. That's the same shape as discovery in Expire discovery queues of stopped agents (#650) #653.
  • A status update is only useful while the caller's in-memory A2ATaskTracker still knows the task, and a restart clears that.
  • Not Ephemeral: deleting the shared queue on dispose would break the replacement pod during a rolling restart.
• Audit: agent.task.status is the only shared fan-out topic on the host topic list. Every other topic is either point-to-point (*.{agentName}) or a work queue (script.invoke), where a pending request should wait, not expire.
• design/messaging.md: the audit note now describes the fix instead of a follow-up, and the cleanup command also lists orphaned *.agent-task-status(.dlq) queues.

Migration

The queue arguments change, so the first start on the new version goes through the existing 406 → delete-and-recreate path, which drops whatever status updates are queued. Those updates are stale anyway: the tracker is empty after the restart. The old rockbot.{Agent}.agent-task-status.dlq is no longer fed and can be deleted once empty.

Test plan

• Full unit suite passes (22 projects).
• RockBot.Messaging.Tests against a local RabbitMQ 4 broker: 50/50.
• New tests:
  • AgentHostSubscriptionTests: options are passed through, defaults are used when no options are set, and the topic's concurrency wins.
  • The builder overload stores the options.
  • The caller registration attaches StatusSubscriptionOptions to the status topic only.
• After deploy: the pod log shows "stale arguments — deleting and recreating" for rockbot.RockBot.agent-task-status, and rabbitmqctl -p / list_queues name consumers arguments | grep agent-task-status shows x-message-ttl 600000, x-expires 86400000, and no dead-letter routing.

Closes #654

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