mq: propagate terminal JetStream consumer errors to the ingest worker #587

Area: mq / ingest — follow-up from CodeRabbit on #586

jsConsumer.Consume (internal/mq/embedded.go) does not pass jetstream.ConsumeErrHandler. In nats.go v1.53.1 the pull consumer reports terminal errors such as ErrConsumerDeleted through that handler and then stops the subscription. The ingest worker's dispatchLoop (internal/ingest/worker.go) only sees messages, so after the client-side buffer drains it sits on msgChan forever without knowing that delivery has stopped. Ingest would silently halt with no log line and no restart.

This predates #586: the worker never passed the handler when it called nats.go directly either. It is deferred out of that PR because it is a design change to the mq.Consumer contract, not a boundary move.

Exposure today

Narrow. The broker is in-process with DontListen, so there is no connection to drop and no external client to delete the buffer-consumer durable. It becomes real once the boundary is backed by a remote broker or once anything in-process can delete or recreate consumers (multi-tenant per-tenant streams, #583 story 5).

Proposed shape

• mq.Consumer gains an asynchronous error path (an error channel or an OnError callback alongside Consume), fed from jetstream.ConsumeErrHandler. Non-terminal errors (ErrNoHeartbeat, leadership changes) are logged; terminal ones (ErrConsumerDeleted, ErrBadRequest, ErrConnectionClosed) are forwarded.
• dispatchLoop selects on that path next to ctx.Done() and msgChan. On a terminal error it logs at ERROR, drains what it has, and either re-creates the consumer and resumes or returns and lets the process fail loud. Which of the two is the decision to make here; fail-loud is consistent with how boot config is treated.
• testutil mock and internal/mq/embedded_test.go coverage for the forwarded error, plus a worker test that a terminal error ends or restarts the loop rather than blocking.

Related: #583 (story 4 sealed the boundary; story 5 changes subjects and streams), #586.