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/src/thread_status.rs

@@ -300,8 +300,16 @@ impl ThreadWatchState {
     }
 
     fn remove_thread(&mut self, thread_id: &str) -> Option<ThreadStatusChangedNotification> {
+        let previous_status = self.status_for(thread_id);
         self.runtime_by_thread_id.remove(thread_id);
-        None
+        if previous_status.is_some() && previous_status != Some(ThreadStatus::NotLoaded) {
+            Some(ThreadStatusChangedNotification {
+                thread_id: thread_id.to_string(),
+                status: ThreadStatus::NotLoaded,
+            })
+        } else {
+            None
+        }
     }
 
     fn update_runtime<F>(
@@ -673,6 +681,15 @@ mod tests {
                 },
             },
         );
+
+        manager.remove_thread(INTERACTIVE_THREAD_ID).await;
+        assert_eq!(
+            recv_status_changed_notification(&mut outgoing_rx).await,
+            ThreadStatusChangedNotification {
+                thread_id: INTERACTIVE_THREAD_ID.to_string(),
+                status: ThreadStatus::NotLoaded,
+            },
+        );
     }
 
     async fn wait_for_status(