agent: Show request usage in the panel (#29006)

This PR adds a banner showing request usage in the Agent panel:

<img width="640" alt="Screenshot 2025-04-17 at 5 51 46 PM"
src="https://github.com/user-attachments/assets/e0eb036c-57c1-441c-bbab-7dab1c6e56d9"
/>

Only visible to users on the new billing.

Note to Joseph: Doesn't need to be cherry-picked to Preview.

Release Notes:

- N/A

---------

Co-authored-by: Nate <nate@zed.dev>

crates/eval/src/example.rs

@@ -421,7 +421,8 @@ impl Example {
                             ThreadEvent::MessageDeleted(_) |
                             ThreadEvent::SummaryChanged |
                             ThreadEvent::SummaryGenerated |
-                            ThreadEvent::CheckpointChanged => {
+                            ThreadEvent::CheckpointChanged |
+                            ThreadEvent::UsageUpdated(_) => {
                                 if std::env::var("ZED_EVAL_DEBUG").is_ok() {
                                     println!("{}Event: {:#?}", log_prefix, event);
                                 }