fix: skip third_party_auth settings tests under CMS and guard enterprise pipeline

The settings tests were failing when run with CMS settings because the
third_party_auth settings (SOCIAL_AUTH_PIPELINE, ExceptionMiddleware,
etc.) are now defined only in lms/envs/common.py.

Investigation confirmed CMS does not need these settings:

- CMS has no social auth URL endpoints (third_party_auth URLs only
  included in LMS when ENABLE_THIRD_PARTY_AUTH is true)
- CMS uses EdxDjangoStrategy for OAuth2 SSO with LMS, not
  ConfigurationModelStrategy for third-party identity providers
- CMS authentication backends are EdXOAuth2 (LMS SSO) and
  LtiAuthenticationBackend, not social auth backends
- The third_party_auth app is only in cms/envs/test.py INSTALLED_APPS
  to avoid import errors from indirect dependencies (like enterprise)

Changes:
- Added @skip_unless_lms decorator to SettingsUnitTest class
- Added hasattr guard for SOCIAL_AUTH_PIPELINE in apps.py to prevent
  AttributeError when running under CMS (which doesn't have this setting)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: feanil

common/djangoapps/third_party_auth/apps.py

@@ -16,5 +16,7 @@ def ready(self):
         # However, the enterprise pipeline step must be inserted dynamically because
         # it requires checking if enterprise is enabled, which can't be done at
         # settings load time.
-        from openedx.features.enterprise_support.api import insert_enterprise_pipeline_elements
-        insert_enterprise_pipeline_elements(settings.SOCIAL_AUTH_PIPELINE)
+        # Only insert enterprise elements if SOCIAL_AUTH_PIPELINE exists (LMS only, not CMS).
+        if hasattr(settings, 'SOCIAL_AUTH_PIPELINE'):
+            from openedx.features.enterprise_support.api import insert_enterprise_pipeline_elements
+            insert_enterprise_pipeline_elements(settings.SOCIAL_AUTH_PIPELINE)

common/djangoapps/third_party_auth/tests/test_settings.py

@@ -5,8 +5,10 @@
 
 from common.djangoapps.third_party_auth import provider
 from common.djangoapps.third_party_auth.tests.utils import skip_unless_thirdpartyauth
+from openedx.core.djangolib.testing.utils import skip_unless_lms
 
 
+@skip_unless_lms
 class SettingsUnitTest(TestCase):
     """Unit tests for third-party auth settings defined in lms/envs/common.py."""