agent: Improve error and warnings display (#36425)

This PR refactors the callout component and improves how we display
errors and warnings in the agent panel, along with improvements for
specific cases (e.g., you have `zed.dev` as your LLM provider and is
signed out).

Still a work in progress, though, wrapping up some details.

Release Notes:

- N/A

crates/agent_ui/src/active_thread.rs

@@ -2597,7 +2597,7 @@ impl ActiveThread {
             .id(("message-container", ix))
             .py_1()
             .px_2p5()
-            .child(Banner::new().severity(ui::Severity::Warning).child(message))
+            .child(Banner::new().severity(Severity::Warning).child(message))
     }
 
     fn render_message_thinking_segment(