Test-Driven Development Philosophy Core principle : Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't. Good tests are integration-style: they exercise real code paths through public APIs. They describe what the system does, not how it does it. A good test reads like a specification - "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure. Bad tests are coupled to implementation. They mock internal collaborators, test…