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_store.rs

@@ -36,8 +36,9 @@ impl ContextStore {
         self.context.push(Context {
             id: self.next_context_id.post_inc(),
             name: name.into(),
-            kind,
+            kind: kind.clone(),
             text: text.into(),
+            icon: kind.icon(),
         });
     }