assistant2: Do not allow a context entry to be added multiple times (#22712)

https://github.com/user-attachments/assets/81674c88-031b-4d55-b362-43819492b93d


Release Notes:

- N/A

crates/assistant2/src/ui/context_pill.rs

@@ -33,10 +33,10 @@ impl RenderOnce for ContextPill {
             px(4.)
         };
         let icon = match self.context.kind {
-            ContextKind::File(_) => IconName::File,
+            ContextKind::File => IconName::File,
             ContextKind::Directory => IconName::Folder,
             ContextKind::FetchedUrl => IconName::Globe,
-            ContextKind::Thread(_) => IconName::MessageCircle,
+            ContextKind::Thread => IconName::MessageCircle,
         };
 
         h_flex()