agent_ui: Allow to configure a default model for profiles through modal (#42359)

Follow-up to https://github.com/zed-industries/zed/pull/39220

This PR allows to configure a default model for a given profile through
the profile management modal.

| Option In Picker | Model Selector |
|--------|--------|
| <img width="1172" height="538" alt="Screenshot 2025-11-10 at 12  24
2@2x"
src="https://github.com/user-attachments/assets/33dfb6f1-f8fd-42f9-b824-3dab807094da"
/> | <img width="1172" height="1120" alt="Screenshot 2025-11-10 at 12 
24@2x"
src="https://github.com/user-attachments/assets/50360b0a-fbb1-455e-9cf7-9fa987345038"
/> |

Release Notes:

- N/A

crates/agent_ui/src/text_thread_editor.rs

@@ -314,6 +314,7 @@ impl TextThreadEditor {
                             )
                         });
                     },
+                    true, // Use popover styles for picker
                     window,
                     cx,
                 )