Inline assistant v2 layout (#22305)

Makes the inline assistant look like @danilo-leal's prototype.

Also fixes the bug with indent guides that @maxdeviant found!

<img width="1059" alt="Screenshot 2024-12-20 at 4 24 56 PM"
src="https://github.com/user-attachments/assets/5e55a3c2-768a-4e9d-bad5-d4ebbe9db9ce"
/>

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -1,4 +1,5 @@
 mod active_thread;
+mod assistant_model_selector;
 mod assistant_panel;
 mod assistant_settings;
 mod buffer_codegen;

crates/assistant2/src/assistant_model_selector.rs

@@ -0,0 +1,85 @@
+use fs::Fs;
+use gpui::View;
+use language_model::LanguageModelRegistry;
+use language_model_selector::{LanguageModelSelector, LanguageModelSelectorPopoverMenu};
+use settings::update_settings_file;
+use std::sync::Arc;
+use ui::{prelude::*, ButtonLike, PopoverMenuHandle, Tooltip};
+
+use crate::{assistant_settings::AssistantSettings, ToggleModelSelector};
+
+pub struct AssistantModelSelector {
+    selector: View<LanguageModelSelector>,
+    menu_handle: PopoverMenuHandle<LanguageModelSelector>,
+}
+
+impl AssistantModelSelector {
+    pub(crate) fn new(
+        fs: Arc<dyn Fs>,
+        menu_handle: PopoverMenuHandle<LanguageModelSelector>,
+        cx: &mut WindowContext,
+    ) -> Self {
+        Self {
+            selector: cx.new_view(|cx| {
+                let fs = fs.clone();
+                LanguageModelSelector::new(
+                    move |model, cx| {
+                        update_settings_file::<AssistantSettings>(
+                            fs.clone(),
+                            cx,
+                            move |settings, _cx| settings.set_model(model.clone()),
+                        );
+                    },
+                    cx,
+                )
+            }),
+            menu_handle,
+        }
+    }
+}
+
+impl Render for AssistantModelSelector {
+    fn render(&mut self, cx: &mut ViewContext<Self>) -> impl IntoElement {
+        let active_model = LanguageModelRegistry::read_global(cx).active_model();
+        let focus_handle = self.selector.focus_handle(cx).clone();
+
+        LanguageModelSelectorPopoverMenu::new(
+            self.selector.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(match active_model {
+                                    Some(model) => h_flex()
+                                        .child(
+                                            Label::new(model.name().0)
+                                                .size(LabelSize::Small)
+                                                .color(Color::Muted),
+                                        )
+                                        .into_any_element(),
+                                    _ => Label::new("No model selected")
+                                        .size(LabelSize::Small)
+                                        .color(Color::Muted)
+                                        .into_any_element(),
+                                }),
+                        )
+                        .child(
+                            Icon::new(IconName::ChevronDown)
+                                .color(Color::Muted)
+                                .size(IconSize::XSmall),
+                        ),
+                )
+                .tooltip(move |cx| {
+                    Tooltip::for_action_in("Change Model", &ToggleModelSelector, &focus_handle, cx)
+                }),
+        )
+        .with_handle(self.menu_handle.clone())
+    }
+}