agent_ui: Do not show token limit callout for external agents (#54090)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

crates/agent_ui/src/conversation_view/thread_view.rs

@@ -8760,7 +8760,7 @@ impl ThreadView {
     }
 
     fn render_token_limit_callout(&self, cx: &mut Context<Self>) -> Option<Callout> {
-        if self.token_limit_callout_dismissed {
+        if self.token_limit_callout_dismissed || self.as_native_thread(cx).is_none() {
             return None;
         }