ui: Implement ParentElement for Banner (#29834)

This PR implements the `ParentElement` trait for the `Banner` component
so that it can use the real children APIs instead of a bespoke one.

Release Notes:

- N/A

crates/agent/src/ui/usage_banner.rs

@@ -66,7 +66,7 @@ impl RenderOnce for UsageBanner {
             }),
         };
 
-        Banner::new().severity(severity).children(
+        Banner::new().severity(severity).child(
             h_flex().flex_1().gap_1().child(Label::new(message)).child(
                 h_flex()
                     .flex_1()