assistant2: Fix filtering issue when using @mention completion provider (#27541)

Previously `src` would not show up because it was filtered out:

<img width="466" alt="image"
src="https://github.com/user-attachments/assets/f3802660-ad73-44be-967d-c332466d9aba"
/>

Release Notes:

- N/A

crates/assistant2/src/context_picker/completion_provider.rs

@@ -544,6 +544,10 @@ impl CompletionProvider for ContextPickerCompletionProvider {
     fn sort_completions(&self) -> bool {
         false
     }
+
+    fn filter_completions(&self) -> bool {
+        false
+    }
 }
 
 fn confirm_completion_callback(