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/active_thread.rs

@@ -22,7 +22,7 @@ pub struct ActiveThread {
     workspace: WeakView<Workspace>,
     language_registry: Arc<LanguageRegistry>,
     tools: Arc<ToolWorkingSet>,
-    pub(crate) thread: Model<Thread>,
+    thread: Model<Thread>,
     messages: Vec<MessageId>,
     list_state: ListState,
     rendered_messages_by_id: HashMap<MessageId, View<Markdown>>,
@@ -68,6 +68,10 @@ impl ActiveThread {
         this
     }
 
+    pub fn thread(&self) -> &Model<Thread> {
+        &self.thread
+    }
+
     pub fn is_empty(&self) -> bool {
         self.messages.is_empty()
     }