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/assistant/src/terminal_inline_assistant.rs

@@ -662,6 +662,7 @@ impl Render for PromptEditor {
                                 cx,
                             )
                         },
+                        gpui::Corner::TopRight,
                     ))
                     .children(
                         if let CodegenStatus::Error(error) = &self.codegen.read(cx).status {