refactor(tests): replace complex TestCaseSource objects with enum-based provider + factory

[DevD4v3]

NUnit occasionally failed in GitHub Actions with a NullReferenceException in CreatePlayer tests when using RepositoryManagerTestCases. The root cause was NUnit’s internal cloning/serialization behavior: complex objects returned by
TestCaseSource can be cloned using GetUninitializedObject, bypassing their constructors and producing partially-initialized instances (e.g., DI-managed repositories set to null).
This happens before the test lifecycle begins and outside the test context, which explains why failures were intermittent and
only reproduced in CI.

This refactor replaces complex test case objects with a simple enum (DatabaseProvider). IRepositoryManager instances are now created inside the test via a dedicated factory, guaranteeing:

• Objects are instantiated within the test’s lifecycle.
• NUnit no longer clones or serializes DI-based classes.
• Stable behavior across local runs and GitHub Actions.
• Proper debugging, since constructors now execute predictably.
• Fully isolated tests with no pre-initialized or shared state.

All tests were updated to use the new enum-based provider and factory pattern.