feat(app-server): thread/unsubscribe API (#10954)

Adds a new v2 app-server API for a client to be able to unsubscribe to a
thread:
- New RPC method: `thread/unsubscribe`
- New server notification: `thread/closed`

Today clients can start/resume/archive threads, but there wasn’t a way
to explicitly unload a live thread from memory without archiving it.
With `thread/unsubscribe`, a client can indicate it is no longer
actively working with a live Thread. If this is the only client
subscribed to that given thread, the thread will be automatically closed
by app-server, at which point the server will send `thread/closed` and
`thread/status/changed` with `status: notLoaded` notifications.

This gives clients a way to prevent long-running app-server processes
from accumulating too many thread (and related) objects in memory.

Closed threads will also be removed from `thread/loaded/list`.

codex-rs/app-server-protocol/schema/json/v2/ThreadUnsubscribeResponse.json

@@ -0,0 +1,23 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "ThreadUnsubscribeStatus": {
+      "enum": [
+        "notLoaded",
+        "notSubscribed",
+        "unsubscribed"
+      ],
+      "type": "string"
+    }
+  },
+  "properties": {
+    "status": {
+      "$ref": "#/definitions/ThreadUnsubscribeStatus"
+    }
+  },
+  "required": [
+    "status"
+  ],
+  "title": "ThreadUnsubscribeResponse",
+  "type": "object"
+}