feat: add spring-boot-testing skill for Spring Boot 4.0 (#1085)

- Introduced MockMvcTester for AssertJ-style assertions in Spring MVC testing.
- Added @RestClientTest for testing REST clients with MockRestServiceServer.
- Implemented RestTestClient as a modern alternative to TestRestTemplate.
- Documented migration steps from Spring Boot 3.x to 4.0, including dependency and annotation changes.
- Created an overview of test slices to guide testing strategies.
- Included Testcontainers setup for JDBC testing with PostgreSQL and MySQL.
- Enhanced @WebMvcTest documentation with examples for various HTTP methods and validation.

skills/spring-boot-testing/references/sb4-migration.md

@@ -0,0 +1,181 @@
+# Spring Boot 4.0 Migration
+
+Key testing changes when migrating from Spring Boot 3.x to 4.0.
+
+## Dependency Changes
+
+### Modular Test Starters
+
+Spring Boot 4.0 introduces modular test starters:
+
+**Before (3.x):**
+
+```xml
+<dependency>
+  <groupId>org.springframework.boot</groupId>
+  <artifactId>spring-boot-starter-test</artifactId>
+  <scope>test</scope>
+</dependency>
+```
+
+**After (4.0) - WebMvc Testing:**
+
+```xml
+<dependency>
+  <groupId>org.springframework.boot</groupId>
+  <artifactId>spring-boot-starter-webmvc-test</artifactId>
+  <scope>test</scope>
+</dependency>
+```
+
+**After (4.0) - REST Client Testing:**
+
+```xml
+<dependency>
+  <groupId>org.springframework.boot</groupId>
+  <artifactId>spring-boot-starter-restclient-test</artifactId>
+  <scope>test</scope>
+</dependency>
+```
+
+## Annotation Migration
+
+### @MockBean → @MockitoBean
+
+**Deprecated (3.x):**
+
+```java
+@MockBean
+private OrderService orderService;
+```
+
+**New (4.0):**
+
+```java
+@MockitoBean
+private OrderService orderService;
+```
+
+### @SpyBean → @MockitoSpyBean
+
+**Deprecated (3.x):**
+
+```java
+@SpyBean
+private PaymentGatewayClient paymentClient;
+```
+
+**New (4.0):**
+
+```java
+@MockitoSpyBean
+private PaymentGatewayClient paymentClient;
+```
+
+## New Testing Features
+
+### RestTestClient
+
+Replaces TestRestTemplate (deprecated):
+
+```java
+@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
+@AutoConfigureRestTestClient
+class OrderIntegrationTest {
+  
+  @Autowired
+  private RestTestClient restClient;
+  
+  @Test
+  void shouldCreateOrder() {
+    restClient
+      .post()
+      .uri("/orders")
+      .body(new OrderRequest("Product", 2))
+      .exchange()
+      .expectStatus()
+      .isCreated()
+      .expectHeader()
+      .location("/orders/1");
+  }
+}
+```
+
+## JUnit 6 Support
+
+Spring Boot 4.0 uses JUnit 6 by default:
+
+- JUnit 4 is deprecated (use JUnit Vintage temporarily)
+- All JUnit 5 features still work
+- Remove JUnit 4 dependencies for clean migration
+
+## Testcontainers 2.0
+
+Module naming changed:
+
+**Before (1.x):**
+
+```xml
+<artifactId>postgresql</artifactId>
+```
+
+**After (2.0):**
+
+```xml
+<artifactId>testcontainers-postgresql</artifactId>
+```
+
+## Non-Singleton Bean Mocking
+
+Spring Framework 7 allows mocking prototype-scoped beans:
+
+```java
+@Component
+@Scope("prototype")
+public class OrderProcessor { }
+
+@SpringBootTest
+class OrderServiceTest {
+  @MockitoBean
+  private OrderProcessor orderProcessor; // Now works!
+}
+```
+
+## SpringExtension Context Changes
+
+Extension context is now test-method scoped by default.
+
+If tests fail with @Nested classes:
+
+```java
+@SpringExtensionConfig(useTestClassScopedExtensionContext = true)
+@SpringBootTest
+class OrderTest {
+  // Use old behavior
+}
+```
+
+## Migration Checklist
+
+- [ ] Replace @MockBean with @MockitoBean
+- [ ] Replace @SpyBean with @MockitoSpyBean
+- [ ] Update Testcontainers dependencies to 2.0 naming
+- [ ] Add modular test starters as needed
+- [ ] Migrate TestRestTemplate to RestTestClient
+- [ ] Remove JUnit 4 dependencies
+- [ ] Update custom TestExecutionListener implementations
+- [ ] Test @Nested class behavior
+
+## Backward Compatibility
+
+Use "classic" starters for gradual migration:
+
+```xml
+<dependency>
+  <groupId>org.springframework.boot</groupId>
+  <artifactId>spring-boot-starter-test-classic</artifactId>
+  <scope>test</scope>
+</dependency>
+```
+
+This provides old behavior while you migrate incrementally.