fix: gate plugin_settings on ENABLE_ENTERPRISE_INTEGRATION

The main behavior change:
- Skip social auth pipeline injection when enterprise integration
  is disabled.
  - This fixes a broken CMS test in the edxapp which expects NO
    enterprise steps in the social auth pipeline.

Other changes rolled into this commit to simplify testing and prep for
making enterprise a truly optional plugin:

- Migrate enterprise FEATURES-dict reads to top-level settings,
  matching openedx-platform's ongoing FEATURES flattening/removal.
- Reorganize enterprise/settings/test.py and invoke plugin_settings
  (via add_plugins()) to prevent future duplication of test settings
  once we migrate enterprise-specific settings into this repo.

ENT-11566

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>