feat: implemeting user finder tests

[lucaspaiva-lp]

Description

This PR fixes multiple issues in the user management and testing modules. It ensures that:

• Exception messages are grammatically correct and consistent ("User is not registered!").
• Unit tests correctly mock repository behavior without breaking logic.
• Tests for UserCreator and UserFinder now properly assert expected responses.
• pytest.raises assertions use .value to access exception messages, resolving ExceptionInfo comparison errors.
• Internal mocks track attributes correctly for assertion checks.

Why

These changes improve test reliability, maintain code consistency, and allow the system to be safely extended for future business logic without introducing test flakiness.

Notes

• No business logic was altered.
• Tests now fully reflect the intended behavior of the repository-controller interaction.
• This PR aligns with the project's clean code principles and layered MVC architecture.