ArcpClient.subscribe leaks per-call resources and double-subscribes on repeat calls for the same job #59

The subscribe method in arcp-client/src/main/java/dev/arcp/client/ArcpClient.java around line 169 creates the per-job SubmissionPublisher via computeIfAbsent and then sends a JobSubscribe envelope to the runtime — but the JOB_SUBSCRIBE send happens unconditionally on every call, not only on the first. A caller that calls subscribe(jobId, opts) twice for the same job receives the same publisher instance (good) but the runtime receives two subscribe envelopes. The runtime's SessionLoop.handleSubscribe in arcp-runtime/src/main/java/dev/arcp/runtime/session/SessionLoop.java around line 719 adds a fresh Subscriber to rec.subscribers() each time, so the second subscribe causes every subsequent job event to fire sub.session().sendJobMessage(...) twice — the consumer sees duplicated events. There is also no way to unsubscribe locally without also unsubscribing on the runtime, which JobUnsubscribe does not address (it removes the server-side mapping but leaves the local SubmissionPublisher alive).\n\nFix prompt: In arcp-client/src/main/java/dev/arcp/client/ArcpClient.java rewrite subscribe so the JOB_SUBSCRIBE wire send only fires when computeIfAbsent actually inserted a new publisher — capture that via an AtomicBoolean updated inside the computeIfAbsent lambda, or pre-check with putIfAbsent and only send when the prior was null. While there, expose a public unsubscribe(JobId) that closes the local SubmissionPublisher and sends JobUnsubscribe to the runtime. In arcp-runtime/src/main/java/dev/arcp/runtime/session/SessionLoop.java make handleSubscribe idempotent: before adding a Subscriber, check whether one already exists for this jobId/session pair, and if so refresh its lastSeenSeq instead of adding a duplicate. Add JUnit tests in arcp-client/src/test covering: two calls to subscribe(jobId, opts) return identical publishers and the consumer sees each event exactly once; unsubscribe closes the local publisher and stops further deliveries.