assistant: Preserve selection focus in the model selector (#23713)

This PR fixes an incorrect behavior in the model selector where we were
removing the focus from the selected item back to the very first item of
the list. Now, if you click/hit return on an item, focus is preserved
there. In other words, focus is always initially in the selected item.

### Before

https://github.com/user-attachments/assets/62b72b1f-4e32-4b4a-adff-dcf9a2c13a28

### After

https://github.com/user-attachments/assets/a8528933-da01-481a-96f3-0173a39a03c0

Release Notes:

- N/A

crates/language_model_selector/src/language_model_selector.rs

@@ -197,6 +197,7 @@ impl PickerDelegate for LanguageModelPickerDelegate {
         cx: &mut Context<Picker<Self>>,
     ) -> Task<()> {
         let all_models = self.all_models.clone();
+        let current_index = self.selected_index;
 
         let llm_registry = LanguageModelRegistry::global(cx);
 
@@ -243,18 +244,27 @@ impl PickerDelegate for LanguageModelPickerDelegate {
 
             this.update_in(&mut cx, |this, window, cx| {
                 this.delegate.filtered_models = filtered_models;
-                this.delegate.set_selected_index(0, window, cx);
+                // Preserve selection focus
+                let new_index = if current_index >= this.delegate.filtered_models.len() {
+                    0
+                } else {
+                    current_index
+                };
+                this.delegate.set_selected_index(new_index, window, cx);
                 cx.notify();
             })
             .ok();
         })
     }
 
-    fn confirm(&mut self, _secondary: bool, _: &mut Window, cx: &mut Context<Picker<Self>>) {
+    fn confirm(&mut self, _secondary: bool, window: &mut Window, cx: &mut Context<Picker<Self>>) {
         if let Some(model_info) = self.filtered_models.get(self.selected_index) {
             let model = model_info.model.clone();
             (self.on_model_changed)(model.clone(), cx);
 
+            let current_index = self.selected_index;
+            self.set_selected_index(current_index, window, cx);
+
             cx.emit(DismissEvent);
         }
     }