Testing the 'Ports' with Spy/Stub.

Use Stubs to provide controlled input to the Use Case and Spies to verify the Use Case's output to the Port.

When testing a Use Case, you don't use a real database. You provide a 'Stub' of the Repository that returns specific entities. You can also use a 'Spy' to ensure that the save method was called with the correct data after the business logic ran.