refactor: migrate ENABLE_THIRD_PARTY_AUTH test usages off FEATURES-as-dict

The ENABLE_THIRD_PARTY_AUTH production readers already read flat
settings.ENABLE_THIRD_PARTY_AUTH (lms/urls.py, oauth_dispatch/urls.py). Remaining test
usages:
- test_login.py / test_register.py enabled it via patch.dict(settings.FEATURES, {...}),
  now override_settings(ENABLE_THIRD_PARTY_AUTH=True).
- third_party_auth/tests/specs/base.py gated IntegrationTest with
  "AUTH_FEATURES_KEY in django_settings.FEATURES"; switched to the flat-based
  testutil.AUTH_FEATURE_ENABLED (hasattr(settings, 'ENABLE_THIRD_PARTY_AUTH')), matching
  the pattern already used in third_party_auth/tests/test_views.py.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>