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/context.rs

@@ -1,6 +1,7 @@
 use gpui::SharedString;
 use language_model::{LanguageModelRequestMessage, MessageContent};
 use serde::{Deserialize, Serialize};
+use ui::prelude::*;
 use util::post_inc;
 
 #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy, Serialize, Deserialize)]
@@ -19,6 +20,7 @@ pub struct Context {
     pub name: SharedString,
     pub kind: ContextKind,
     pub text: SharedString,
+    pub icon: IconName,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
@@ -29,6 +31,17 @@ pub enum ContextKind {
     Thread,
 }
 
+impl ContextKind {
+    pub fn icon(&self) -> IconName {
+        match self {
+            ContextKind::File => IconName::File,
+            ContextKind::Directory => IconName::Folder,
+            ContextKind::FetchedUrl => IconName::Globe,
+            ContextKind::Thread => IconName::MessageCircle,
+        }
+    }
+}
+
 pub fn attach_context_to_message(
     message: &mut LanguageModelRequestMessage,
     context: impl IntoIterator<Item = Context>,