[nightly] Fix List[String].__str__() removed: use String.join() instead

Co-authored-by: cursor-agent <cursor-agent@cursor.sh>
Made-with: Cursor

Author: ehsanmok

examples/04_gpu_parsing.mojo

@@ -48,7 +48,8 @@ fn main() raises:
     with open("gpu_test.json", "r") as f:
         var file_data = load[target="gpu"](f)
         print("   Loaded from file successfully!")
-        print("   Object keys:", file_data.object_keys().__str__())
+        var keys = file_data.object_keys()
+        print("   Object keys:", ", ".join(keys))
     print()
 
     # Comparing CPU vs GPU (for demonstration)