assistant2: Improve clarity of loading state (#26178)

Follow up to https://github.com/zed-industries/zed/pull/23299.

Having the loading state on the button makes sense, but it's also too
subtle. If you're waiting on an LLM response that takes a while, like a
"thinking state", not having anything more clearly visible communicating
that the model is still in-progress can make you think something is
wrong.

<img
src="https://github.com/user-attachments/assets/da64516e-5540-4294-97a2-e4542ce704f3"
width="700px" />

Release Notes:

- N/A

crates/assistant2/src/assistant_panel.rs

@@ -1023,12 +1023,7 @@ impl Render for AssistantPanel {
             .map(|parent| match self.active_view {
                 ActiveView::Thread => parent
                     .child(self.render_active_thread_or_empty_state(window, cx))
-                    .child(
-                        h_flex()
-                            .border_t_1()
-                            .border_color(cx.theme().colors().border)
-                            .child(self.message_editor.clone()),
-                    )
+                    .child(h_flex().child(self.message_editor.clone()))
                     .children(self.render_last_error(cx)),
                 ActiveView::History => parent.child(self.history.clone()),
                 ActiveView::PromptEditor => parent.children(self.context_editor.clone()),