feat: add SAML provider admin API and migrate SAML utilities

- Add SAMLProviderConfigViewSet and SAMLProviderDataViewSet to the
  enterprise API with permission checks and comprehensive tests.
- Add get_saml_identity_provider and get_identity_providers utility
  views for enterprise SAML operations.
- Migrate SAML utility functions (get_saml_provider_data_by_enterprise,
  get_saml_provider_configuration) from third_party_auth to enterprise,
  with comprehensive tests using django-mock-queries MockSet.

ENT-11567

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