test(gubbins): add test to verify with_config_repo uses extension Config

This test validates that the with_config_repo fixture uses extension-aware
Config discovery. It checks that the JSON file contains GubbinsSpecificInfo
in user configs, which only exists if gubbins Config was used for serialization.

The test fails without the fix in diracx-testing and passes with it.

Author: chrisburr

extensions/gubbins/gubbins-core/tests/test_config.py

@@ -36,3 +36,33 @@ def test_remote_git_config_source(monkeypatch):
     assert isinstance(modified, datetime.datetime)
     result = remote_conf.read_raw(hexsha, modified)
     assert isinstance(result, Config)
+
+
+def test_with_config_repo_uses_gubbins_config(with_config_repo):
+    """Verify with_config_repo fixture uses gubbins Config, not base Config.
+
+    The with_config_repo fixture must use select_from_extension() to discover
+    the Config class, so extensions like gubbins can add fields to the schema.
+
+    Before the fix (DIRACGrid/diracx#727), the fixture directly imported Config
+    from diracx.core.config, bypassing the extension system. This caused the
+    fixture to serialize config using base UserConfig (without GubbinsSpecificInfo),
+    so the JSON file wouldn't contain gubbins-specific fields.
+
+    This test reads the raw JSON to verify it contains GubbinsSpecificInfo,
+    which proves the fixture used gubbins Config for serialization.
+    """
+    import json
+
+    # Read the raw JSON file created by the fixture
+    config_file = with_config_repo / "default.yml"
+    raw_config = json.loads(config_file.read_text())
+
+    # The JSON should contain GubbinsSpecificInfo in user configs
+    # This field only exists if gubbins Config was used to serialize
+    for vo_config in raw_config["Registry"].values():
+        for user_config in vo_config["Users"].values():
+            assert "GubbinsSpecificInfo" in user_config, (
+                "GubbinsSpecificInfo not found in JSON - fixture may not be using "
+                "extension-aware Config discovery"
+            )