Write unit and integration tests following the project's testing patterns with Jest. Use this skill when creating test files (*.spec.ts), writing unit tests for DTOs and domain entities, writing integration tests for use cases and queries, setting up test fixtures, or using test utilities. This includes organizing tests in test/unit/ and test/integration/ directories, using proper test naming conventions, mocking external dependencies, and testing with real database transactions for integration tests. Apply when working with any *.spec.ts file, test fixtures, or implementing the project's test-behavior-not-implementation philosophy.