Refactor test suite: Prioritize unit test

[antoinebhs]

Our current test suite heavily use integration tests using @SpringBootTest, which makes tests slow, brittle, and harder to maintain. We need to refactor the entire test suite to prioritize pure unit tests (95%) over integration tests (5%) and ensure we only test the code that actually exists in our application.

• Maximize unit test coverage - Replace most @SpringBootTest tests with lightweight @ExtendWith(MockitoExtension.class) tests
  Test only production code - No testing of Spring framework, third-party libraries, or non-existent behaviors
  Improve test speed - Unit tests should execute in < 100ms each
  Increase maintainability - Clear, focused tests that are easy to understand and modify

• Unit Tests First (95% of all tests)
  Characteristics of a proper unit test:
  Tests ONE class in isolation
  Uses @ExtendWith(MockitoExtension.class) (NOT @SpringBootTest)
  Mocks ALL dependencies with @mock and @Injectmocks
  No database, no Spring context, no MockMvc
  Executes in < 100ms
  Example structure:

@ExtendWith(MockitoExtension.class)
class ServiceNameUnitTest {
    
    @Mock
    private DependencyService dependencyService;
    
    @Mock
    private Repository repository;
    
    @InjectMocks
    private ServiceName serviceUnderTest;
    
    @Test
    void shouldReturnExpectedResultWhenValidInput() {
        // Arrange
        InputData input = new InputData("value");
        when(dependencyService.process(any())).thenReturn(expectedData);
        
        // Act
        Result result = serviceUnderTest.methodToTest(input);
        
        // Assert
        assertThat(result).isNotNull()
            .extracting("field")
            .isEqualTo(expectedValue);
        verify(dependencyService).process(any());
    }
}

• Integration Tests (5% maximum)
  Only use when:
  Testing critical cross-component interactions
  Testing custom repository queries
  End-to-end controller testing (use @WebMvcTest when possible, not @SpringBootTest)

• Test Only What Exists
  To do:
  Analyze the production class before writing tests
  Test each public method
  Cover conditional branches (if/else, switch)
  Test error cases (exceptions, validations)
  Test edge cases and boundary conditions
  Not to do:
  Test unimplemented behaviors
  Test methods from other microservices/libraries
  Test Spring infrastructure (dependency injection, transactions)
  Test frameworks (JPA, Jackson, Spring MVC)
  Test multiple classes together in unit tests
  Test private methods directly