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/tests/common/mcp_process.rs

@@ -66,6 +66,7 @@ use codex_app_server_protocol::ThreadRollbackParams;
 use codex_app_server_protocol::ThreadSetNameParams;
 use codex_app_server_protocol::ThreadStartParams;
 use codex_app_server_protocol::ThreadUnarchiveParams;
+use codex_app_server_protocol::ThreadUnsubscribeParams;
 use codex_app_server_protocol::TurnCompletedNotification;
 use codex_app_server_protocol::TurnInterruptParams;
 use codex_app_server_protocol::TurnStartParams;
@@ -433,6 +434,15 @@ impl McpProcess {
         self.send_request("thread/name/set", params).await
     }
 
+    /// Send a `thread/unsubscribe` JSON-RPC request.
+    pub async fn send_thread_unsubscribe_request(
+        &mut self,
+        params: ThreadUnsubscribeParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("thread/unsubscribe", params).await
+    }
+
     /// Send a `thread/unarchive` JSON-RPC request.
     pub async fn send_thread_unarchive_request(
         &mut self,