assistant2: Expose ActiveThread::thread via a getter (#23577)

This PR exposes the `thread` file on the `ActiveThread` via a getter
rather than exposing the field directly.

Release Notes:

- N/A

crates/assistant2/src/assistant_panel.rs

@@ -432,7 +432,7 @@ impl AssistantPanel {
     }
 
     pub(crate) fn active_thread(&self, cx: &AppContext) -> Model<Thread> {
-        self.thread.read(cx).thread.clone()
+        self.thread.read(cx).thread().clone()
     }
 
     pub(crate) fn delete_thread(&mut self, thread_id: &ThreadId, cx: &mut ViewContext<Self>) {