assistant2: Ensure errors are also displayed in populated new thread view (#27869)

Follow-up to https://github.com/zed-industries/zed/pull/27812

This PR makes sure these errors cases also show up in the panel's empty
state even when there is past data.

| No ToS | Missing Provider |
|--------|--------|
| ![CleanShot 2025-04-01 at 4  49
36@2x](https://github.com/user-attachments/assets/6da6bdc9-daa6-4a7b-a224-989eb845e205)
| ![CleanShot 2025-04-01 at 4  50
04@2x](https://github.com/user-attachments/assets/bddf62cb-3727-44b5-b115-9a88313c6d85)
|

Release Notes:

- N/A

crates/ui/src/components/banner.rs

@@ -130,7 +130,7 @@ impl RenderOnce for Banner {
                 .child(content_area)
                 .child(action_slot);
         } else {
-            container = container.px_2().child(content_area);
+            container = container.px_2().child(div().w_full().child(content_area));
         }
 
         container