Adjust model selector popover design (#15056)

This PR mostly refines the model selector popover design by formatting
the models names' and adjusting spacing/alignment in the list-related
items. The list component changes could've been made in a separate PR
but it was also very practical to do it here as I was already
in-context. Either way, I'm happy to separate if that's better!

One thing I couldn't necessarily figure out, though, is why the order
changed (e.g., Anthropic at last ). I wonder if that was because of the
separator logic somehow? I'd love guidance here—new to Rust!

| Before | After |
|--------|--------|
| <img width="228" alt="Screenshot 2024-07-23 at 21 02 33"
src="https://github.com/user-attachments/assets/3372c6c9-08dc-4d71-9265-26f015e2dbc2">
| <img width="228" alt="Screenshot 2024-07-23 at 21 01 45"
src="https://github.com/user-attachments/assets/624cc7db-a3d9-48e3-99d7-c29829501130">
|

---

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/assistant/src/assistant_panel.rs

@@ -2630,10 +2630,43 @@ impl Render for ContextEditorToolbarItem {
             });
         let right_side = h_flex()
             .gap_2()
-            .child(ModelSelector::new(
-                self.model_selector_menu_handle.clone(),
-                self.fs.clone(),
-            ))
+            .child(
+                ModelSelector::new(
+                    self.fs.clone(),
+                    ButtonLike::new("active-model")
+                        .style(ButtonStyle::Subtle)
+                        .child(
+                            h_flex()
+                                .w_full()
+                                .gap_0p5()
+                                .child(
+                                    div()
+                                        .overflow_x_hidden()
+                                        .flex_grow()
+                                        .whitespace_nowrap()
+                                        .child(
+                                            Label::new(
+                                                LanguageModelCompletionProvider::read_global(cx)
+                                                    .active_model()
+                                                    .map(|model| model.name().0)
+                                                    .unwrap_or_else(|| "No model selected".into()),
+                                            )
+                                            .size(LabelSize::Small)
+                                            .color(Color::Muted),
+                                        ),
+                                )
+                                .child(
+                                    Icon::new(IconName::ChevronDown)
+                                        .color(Color::Muted)
+                                        .size(IconSize::XSmall),
+                                ),
+                        )
+                        .tooltip(move |cx| {
+                            Tooltip::for_action("Change Model", &ToggleModelSelector, cx)
+                        }),
+                )
+                .with_handle(self.model_selector_menu_handle.clone()),
+            )
             .children(self.render_remaining_tokens(cx))
             .child(self.render_inject_context_menu(cx));