assistant2: Derive the Context icon at render time instead of storing (#22397)

This PR is a follow up to #22385 that makes it so we treat the icon as
derived state.

Release Notes:

- N/A

crates/assistant2/src/context_store.rs

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