assistant: Improve the language model selector (#25125)

This PR includes change such as:

- Ensures the popover width is fixed/not dancing around
- Ensures the popover is not obscuring the trigger in the buffer and
terminal inline assistant scenarios
- Removes ellipsis from the trigger button label
- Ensures the scrollbar doesn't hide the check icon

| Terminal | Prompt Editor | Buffer |
|--------|--------|--------|
| ![Screenshot 2025-02-18 at 8 43
46 PM](https://github.com/user-attachments/assets/9cdfbaf1-f27e-4f48-877e-9cf61767ecee)
| ![Screenshot 2025-02-18 at 8 43
49 PM](https://github.com/user-attachments/assets/7abf9be2-bd2a-43d7-9a5d-d665e7e9fda3)
| ![Screenshot 2025-02-18 at 8 43
52 PM](https://github.com/user-attachments/assets/017bbdb3-185a-4bf6-9005-018ecafef9dd)
|

Release Notes:

- N/A

crates/assistant2/src/assistant_model_selector.rs

@@ -61,13 +61,9 @@ impl Render for AssistantModelSelector {
                     h_flex()
                         .gap_0p5()
                         .child(
-                            div().max_w_32().child(
-                                Label::new(model_name)
-                                    .size(LabelSize::Small)
-                                    .color(Color::Muted)
-                                    .text_ellipsis()
-                                    .into_any_element(),
-                            ),
+                            Label::new(model_name)
+                                .size(LabelSize::Small)
+                                .color(Color::Muted),
                         )
                         .child(
                             Icon::new(IconName::ChevronDown)
@@ -84,6 +80,7 @@ impl Render for AssistantModelSelector {
                     cx,
                 )
             },
+            gpui::Corner::BottomRight,
         )
         .with_handle(self.menu_handle.clone())
     }