agent: Show provider icon in model selectors (#30595)

I often switch between models, and I believe many people do. Currently,
it is difficult to determine which provider offers the selected model
because the same models are available from different providers. I
propose a simple change to the selector so that users can distinguish
between providers from the model they have chosen.

As a side note, I would actually prefer to have a text label with the
provider’s name next to the model name in the selector. However, I
understand that this is too opinionated and takes up too much space.

| Before | After |
| ------ | ------ |
|
![before_inline_assist](https://github.com/user-attachments/assets/35617ba5-e8d4-4dab-a997-f7286f73f2bf)
|
![after_inline_assist](https://github.com/user-attachments/assets/c37c81b4-73e4-49e2-955d-b8543b2855ad)
|
|
![before_text_thread](https://github.com/user-attachments/assets/af90303b-12d6-402c-90a5-8b36cd97396e)
|
![after_text_thread](https://github.com/user-attachments/assets/bca5b423-f12b-4eaf-a82e-424d09b7f447)
|
|
![before_thread](https://github.com/user-attachments/assets/0946775f-1d52-437b-a217-9708ee2e789a)
|
![after_thread](https://github.com/user-attachments/assets/f5e53968-9020-446f-9d5e-653ae9fdea3e)
|

Release Notes:

- The model selector has been improved with a provider icon, making it
easier to distinguish between providers.

---------

Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/agent_ui/src/text_thread_editor.rs

@@ -2120,12 +2120,21 @@ impl TextThreadEditor {
         let active_model = LanguageModelRegistry::read_global(cx)
             .default_model()
             .map(|default| default.model);
-        let focus_handle = self.editor().focus_handle(cx).clone();
         let model_name = match active_model {
             Some(model) => model.name().0,
             None => SharedString::from("No model selected"),
         };
 
+        let active_provider = LanguageModelRegistry::read_global(cx)
+            .default_model()
+            .map(|default| default.provider);
+        let provider_icon = match active_provider {
+            Some(provider) => provider.icon(),
+            None => IconName::Ai,
+        };
+
+        let focus_handle = self.editor().focus_handle(cx).clone();
+
         PickerPopoverMenu::new(
             self.language_model_selector.clone(),
             ButtonLike::new("active-model")
@@ -2133,10 +2142,16 @@ impl TextThreadEditor {
                 .child(
                     h_flex()
                         .gap_0p5()
+                        .child(
+                            Icon::new(provider_icon)
+                                .color(Color::Muted)
+                                .size(IconSize::XSmall),
+                        )
                         .child(
                             Label::new(model_name)
+                                .color(Color::Muted)
                                 .size(LabelSize::Small)
-                                .color(Color::Muted),
+                                .ml_0p5(),
                         )
                         .child(
                             Icon::new(IconName::ChevronDown)