Added the `EndpointAuthApiTests` class to test the `EndpointAuth` API.
Tests include:
- Dependency injection resolution (`ReCClient_endpoint_auth_api_is_resolvable_through_dependency_injection`).
- `CreateAsync` behavior with minimal payload.
- `UpdateAsync` behavior with an unknown ID.
- `DeleteAsync` behavior, ensuring payload is sent as a query string.
Introduced necessary `using` directives and utilized the `CreateScopedClient` helper method for scoped client creation. Added exception handling to validate API method behavior and ensure proper assertions on exceptions.
