agent: Fix expand message editor while not focused (#28650)

Allow expanding the message editor while the agent panel is not focused,
right now there is no effect when you use the button from another focus

Release Notes:

- N/A

crates/agent/src/message_editor.rs

@@ -157,7 +157,7 @@ impl MessageEditor {
         cx.notify();
     }
 
-    fn expand_message_editor(
+    pub fn expand_message_editor(
         &mut self,
         _: &ExpandMessageEditor,
         _window: &mut Window,