agent: Only show expand message editor when focused on it (#29595)

Simplifying the UI as much as possible.

Release Notes:

- N/A

crates/agent/src/message_editor.rs

@@ -495,7 +495,8 @@ impl MessageEditor {
                     .items_start()
                     .justify_between()
                     .child(self.context_strip.clone())
-                    .child(
+                    .when(focus_handle.is_focused(window), |this| {
+                        this.child(
                             IconButton::new("toggle-height", expand_icon)
                                 .icon_size(IconSize::XSmall)
                                 .icon_color(Color::Muted)
@@ -520,7 +521,8 @@ impl MessageEditor {
                                 .on_click(cx.listener(|_, _, window, cx| {
                                     window.dispatch_action(Box::new(ExpandMessageEditor), cx);
                                 })),
-                    ),
+                        )
+                    }),
             )
             .child(
                 v_flex()