Refactor UserAuthorizable

What does this MR do and why?

This MR refactors the UserAuthorizable module to transition from using CloudConnector::AvailableServices to the new UnitPrimitive API. This change is part of a larger effort to consolidate and modernize the AI feature authorization system.

Key Changes:

1. Parameter Renaming: Updated method signatures from service_name to unit_primitive_name across the codebase
2. Unit Primitive Integration: Added support for fetching unit primitives from Gitlab::CloudConnector::DataModel::UnitPrimitive catalog
3. Feature Flag Protection: Changes are behind use_unit_primitives_in_user_authorizable feature flag for safe rollout
4. Backward Compatibility: Maintains support for existing AvailableServices when feature flag is disabled
5. Self-hosted Models Support: Added special handling for self-hosted models unit primitive
6. Add-on Purchase Logic: Updated to handle both old service-based and new unit primitive-based add-on structures

References

• Feature Issue: #562379 (closed)
• Rollout Issue: #562380 (closed)

Screenshots or screen recordings

N/A - Backend refactoring with no UI changes

How to set up and validate locally

1. Enable the feature flag in Rails console:

     Feature.enable(:use_unit_primitives_in_user_authorizable)

2. Test AI feature authorization:

     user = User.find_by(username: 'your_username')
     user.allowed_to_use?(:duo_chat, unit_primitive_name: :duo_chat)

3. Verify both code paths work:

   # Test with feature flag enabled (new unit primitive path)
   Feature.enable(:use_unit_primitives_in_user_authorizable)
   user.allowed_to_use?(:duo_chat)
   
   # Test with feature flag disabled (legacy service path)
   Feature.disable(:use_unit_primitives_in_user_authorizable)
   user.allowed_to_use?(:duo_chat)

Note: We need to test:

• SelfHostedModels flow, as this change is affecting it as well. The steps are the same, but SHM needs to be configured.
• generate_description feature, as we moved to UP called generate_issue_description

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

• Feature flag added for safe rollout
• Backward compatibility maintained
• Comprehensive test coverage updated
• No breaking changes to existing API
• Performance impact minimal (same authorization logic, different data source)