unit-test-service-layer

# Unit Testing Service Layer with Mockito ## Overview Provides patterns for unit testing `@Service` classes using Mockito. Mocks repository calls, verifies method invocations, tests exception scenarios, and stubs external API responses. Enables fast, isolated tests without Spring container or database. ## When to Use - Testing business logic in `@Service` classes - Mocking repository and external client dependencies - Verifying service interactions with mocked collaborators - Testing error handling and edge cases in services - Writing fast, isolated unit tests (no database, no API calls) ## Instructions Follow this workflow to test service layer with Mockito, including validation checkpoints: ### 1. Setup Test Class Use `@ExtendWith(MockitoExtension.class)` to enable Mockito annotations. ### 2. Declare Mocks with `@Mock` and `@InjectMocks` Use `@Mock` for dependencies (repositories, clients) and `@InjectMocks` for the service under test. ### 3. Arrange-Act-Assert with Validation **Arrange**: Create test data and configure mock return values using `when().thenReturn()`. **Act**: Execute the service method being tested. **Assert**: - Verify returned values with AssertJ assertions - Verify mock interactions with `verify()` - **Validation checkpoint**: Run test and confirm green bar ### 4. Test Exception Scenarios Configure mocks to throw exceptions with `when().thenThrow()`. **Validation checkpoint**: Verify exception type and message ### 5. Verify Complete Cov...