assistant: Fix toggling the model selector via keybind (#16319)

This PR restores the ability to toggle the model selector via a keybind
after it was lost in #15693.

Release Notes:

- Restored the ability to toggle the model selector in the Assistant via
a keybinding (Preview only).

crates/assistant/src/model_selector.rs

@@ -295,5 +295,6 @@ impl<T: PopoverTrigger> RenderOnce for ModelSelector<T> {
             .menu(move |_cx| Some(picker_view.clone()))
             .trigger(self.trigger)
             .attach(gpui::AnchorCorner::BottomLeft)
+            .when_some(self.handle, |menu, handle| menu.with_handle(handle))
     }
 }