Step-up auth: Add FactoryBot factory for OmniAuth provider configs

What does this MR do and why?

Reduces duplicated GitlabSettings::Options configuration code across 14 step-up authentication test files. The factory centralizes common configuration patterns, removing ~180 lines of repetitive setup code.

This improves maintainability by providing a single source of truth for test configurations, making future changes easier to implement consistently across all step-up auth tests.

🛠️ with ❤️ at Siemens

References

• Related issue: #556943

Screenshots or screen recordings

Not applicable - test code refactoring only.

How to set up and validate locally

1. Run the step-up authentication tests:

   bundle exec rspec \
     spec/factories/omniauth_provider_configs.rb \
     spec/lib/gitlab/auth/oidc/omniauth_provider_configs_factory_spec.rb \
     spec/controllers/concerns/enforces_step_up_authentication_spec.rb \
     spec/controllers/concerns/enforces_step_up_authentication_for_namespace_spec.rb \
     spec/controllers/omniauth_callbacks_controller_spec.rb \
     spec/helpers/auth_helper_spec.rb \
     spec/helpers/groups_helper_spec.rb \
     spec/lib/gitlab/auth/oidc/step_up_auth_before_request_phase_spec.rb \
     spec/lib/gitlab/auth/oidc/step_up_authentication_flow_spec.rb \
     spec/lib/gitlab/auth/oidc/step_up_authentication_spec.rb \
     spec/support/shared_examples/enforce_step_up_auth_shared_examples.rb \
     spec/views/admin/sessions/new.html.haml_spec.rb

2. Verify all 151 tests pass.

3. Review the factory at spec/factories/omniauth_provider_configs.rb for the centralized configuration patterns.

MR acceptance checklist

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