assistant2: Add icons to the context pill (#22385)

This PR adds an icon field to the `ContextKind` enum, which means that
icons will now display on context pills, both on the message editor and
on the active thread.

<img width="800" alt="Screenshot 2024-12-24 at 12 23 17 AM"
src="https://github.com/user-attachments/assets/f00e540b-30fe-49ac-b3df-7c7a5dc86d65"
/>

Release Notes:

- N/A

crates/assistant2/src/ui/context_pill.rs

@@ -27,15 +27,26 @@ impl ContextPill {
 
 impl RenderOnce for ContextPill {
     fn render(self, cx: &mut WindowContext) -> impl IntoElement {
+        let padding_right = if self.on_remove.is_some() {
+            px(2.)
+        } else {
+            px(4.)
+        };
+
         h_flex()
             .gap_1()
-            .pl_1p5()
-            .pr_0p5()
+            .pl_1()
+            .pr(padding_right)
             .pb(px(1.))
             .border_1()
             .border_color(cx.theme().colors().border.opacity(0.5))
             .bg(cx.theme().colors().element_background)
             .rounded_md()
+            .child(
+                Icon::new(self.context.icon)
+                    .size(IconSize::XSmall)
+                    .color(Color::Muted),
+            )
             .child(Label::new(self.context.name.clone()).size(LabelSize::Small))
             .when_some(self.on_remove, |parent, on_remove| {
                 parent.child(