Place Link Agent into maintenance mode

Place a Link Agent into maintenance mode. mabl-side routing immediately stops treating the Agent as an active target — legacy Link Servers no longer dispatch new requests to it, and the Agent's mablnet Host begins draining its carriers (refusing new streams while continuing to service in-flight ones). The Agent's status moves to draining immediately and to drained once all in-flight connections have finished. If drain_timeout_seconds is supplied, the Agent stops waiting for natural drain after that interval and moves to drained regardless; without a timeout it waits indefinitely. The Agent does not auto-shutdown — the final shutdown is left to whatever external orchestrator initiated the maintenance window (use POST .../terminate to stop it, or DELETE this resource to return it to service). Idempotent; re-posting while still draining updates drain_timeout_seconds.