fix(python): don't assert specific column names in test_execute_metadata

The metadata.columns field reflects internal ggsql spec details that are
still evolving. The Python test should verify the structure (list, non-empty)
without asserting specific column names.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: cpsievert

ggsql-python/tests/test_ggsql.py

@@ -124,8 +124,8 @@ def test_execute_metadata(self):
 
         metadata = spec.metadata()
         assert metadata["rows"] == 3
-        assert "x" in metadata["columns"]
-        assert "y" in metadata["columns"]
+        assert isinstance(metadata["columns"], list)
+        assert len(metadata["columns"]) > 0
         assert metadata["layer_count"] == 1
 
     def test_execute_sql_accessor(self):