fix(broker): delete leaked JetStream consumers on rule subject change

[danielmichaels]

Summary

Rule updates that change or remove a subject could leave JetStream durable consumers behind, and failed subscription startup could leave partial state behind. This PR makes those
updates atomic and ensures obsolete consumers are deleted when they are no longer referenced.

What changed

• store both stream name and consumer name per subject so cleanup always targets the original durable consumer
• apply rule updates as try-then-commit: start new NATS and HTTP subscriptions first, abort and roll back on failure, then commit rule state
• remove NATS and outbound subscriptions only when the subject is no longer referenced by any remaining rule key
• drain in-flight workers before deleting durable consumers to avoid Ack/Nak races and duplicate redelivery when the consumer is recreated
• clean up orphaned consumers created by failed subscription or outbound registration attempts
• treat missing consumers as an idempotent cleanup path and log manual cleanup only for real delete failures

Notes

• this does not backfill cleanup for consumers leaked before this patch; existing orphans still need one-time manual cleanup