agent: Improve pickers and their triggers styles in the panel (#40284)

Making all triggers have the same style when the picker is open
(including changing the icon when the picker opens on top of the
trigger). Also removed the footer from the ACP model selector given
there's nothing to consider when that's the case; users can only
configure LLM providers when using Zed's built-in agent.

Release Notes:

- N/A

crates/agent_ui/src/text_thread_editor.rs

@@ -1977,7 +1977,9 @@ impl TextThreadEditor {
             cx.entity().downgrade(),
             IconButton::new("trigger", IconName::Plus)
                 .icon_size(IconSize::Small)
-                .icon_color(Color::Muted),
+                .icon_color(Color::Muted)
+                .selected_icon_color(Color::Accent)
+                .selected_style(ButtonStyle::Filled),
             move |window, cx| {
                 Tooltip::with_meta(
                     "Add Context",
@@ -2052,30 +2054,27 @@ impl TextThreadEditor {
         };
 
         let focus_handle = self.editor().focus_handle(cx);
+        let (color, icon) = if self.language_model_selector_menu_handle.is_deployed() {
+            (Color::Accent, IconName::ChevronUp)
+        } else {
+            (Color::Muted, IconName::ChevronDown)
+        };
 
         PickerPopoverMenu::new(
             self.language_model_selector.clone(),
             ButtonLike::new("active-model")
-                .style(ButtonStyle::Subtle)
+                .selected_style(ButtonStyle::Tinted(TintColor::Accent))
                 .child(
                     h_flex()
                         .gap_0p5()
-                        .child(
-                            Icon::new(provider_icon)
-                                .color(Color::Muted)
-                                .size(IconSize::XSmall),
-                        )
+                        .child(Icon::new(provider_icon).color(color).size(IconSize::XSmall))
                         .child(
                             Label::new(model_name)
-                                .color(Color::Muted)
+                                .color(color)
                                 .size(LabelSize::Small)
                                 .ml_0p5(),
                         )
-                        .child(
-                            Icon::new(IconName::ChevronDown)
-                                .color(Color::Muted)
-                                .size(IconSize::XSmall),
-                        ),
+                        .child(Icon::new(icon).color(color).size(IconSize::XSmall)),
                 ),
             move |window, cx| {
                 Tooltip::for_action_in(
@@ -2086,7 +2085,7 @@ impl TextThreadEditor {
                     cx,
                 )
             },
-            gpui::Corner::BottomLeft,
+            gpui::Corner::BottomRight,
             cx,
         )
         .with_handle(self.language_model_selector_menu_handle.clone())