Add Agent Preview trait (#29760)

Like the title says

Release Notes:

- N/A

crates/agent/src/ui.rs

@@ -1,9 +1,12 @@
 mod agent_notification;
+pub mod agent_preview;
 mod animated_label;
 mod context_pill;
+mod upsell;
 mod usage_banner;
 
 pub use agent_notification::*;
+pub use agent_preview::*;
 pub use animated_label::*;
 pub use context_pill::*;
 pub use usage_banner::*;