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/webmvctest.md

@@ -0,0 +1,177 @@
+# @WebMvcTest
+
+Testing Spring MVC controllers with focused slice tests.
+
+## Basic Structure
+
+```java
+@WebMvcTest(OrderController.class)
+class OrderControllerTest {
+  
+  @Autowired
+  private MockMvcTester mvc;
+  
+  @MockitoBean
+  private OrderService orderService;
+  
+  @MockitoBean
+  private UserService userService;
+}
+```
+
+## What Gets Loaded
+
+- The specified controller(s)
+- Spring MVC infrastructure (HandlerMapping, HandlerAdapter)
+- Jackson ObjectMapper (for JSON)
+- Exception handlers (@ControllerAdvice)
+- Spring Security filters (if on classpath)
+- Validation (if on classpath)
+
+## Testing GET Endpoints
+
+```java
+@Test
+void shouldReturnOrder() {
+  var order = new Order(1L, "PENDING", BigDecimal.valueOf(99.99));
+  given(orderService.findById(1L)).willReturn(order);
+  
+  assertThat(mvc.get().uri("/orders/1"))
+    .hasStatusOk()
+    .hasContentType(MediaType.APPLICATION_JSON)
+    .bodyJson()
+    .extractingPath("$.status")
+    .isEqualTo("PENDING");
+}
+```
+
+## Testing POST with Request Body
+
+### Using Text Blocks (Java 25)
+
+```java
+@Test
+void shouldCreateOrder() {
+  given(orderService.create(any(OrderRequest.class))).willReturn(1L);
+  
+  var json = """
+    {
+      "product": "Product A",
+      "quantity": 2
+    }
+    """;
+  
+  assertThat(mvc.post().uri("/orders")
+    .contentType(MediaType.APPLICATION_JSON)
+    .content(json))
+    .hasStatus(HttpStatus.CREATED)
+    .hasHeader("Location", "/orders/1");
+}
+```
+
+### Using Records
+
+```java
+record OrderRequest(String product, int quantity) {}
+
+@Test
+void shouldCreateOrderWithRecord() {
+  var request = new OrderRequest("Product A", 2);
+  given(orderService.create(any())).willReturn(1L);
+  
+  assertThat(mvc.post().uri("/orders")
+    .contentType(MediaType.APPLICATION_JSON)
+    .content(json.write(request).getJson()))
+    .hasStatus(HttpStatus.CREATED);
+}
+```
+
+## Testing Validation Errors
+
+```java
+@Test
+void shouldRejectInvalidOrder() {
+  var invalidJson = """
+    {
+      "product": "",
+      "quantity": -1
+    }
+    """;
+  
+  assertThat(mvc.post().uri("/orders")
+    .contentType(MediaType.APPLICATION_JSON)
+    .content(invalidJson))
+    .hasStatus(HttpStatus.BAD_REQUEST)
+    .bodyJson()
+    .hasPath("$.errors");
+}
+```
+
+## Testing Query Parameters
+
+```java
+@Test
+void shouldFilterOrdersByStatus() {
+  assertThat(mvc.get().uri("/orders?status=PENDING"))
+    .hasStatusOk();
+  
+  verify(orderService).findByStatus(OrderStatus.PENDING);
+}
+```
+
+## Testing Path Variables
+
+```java
+@Test
+void shouldCancelOrder() {
+  assertThat(mvc.put().uri("/orders/123/cancel"))
+    .hasStatusOk();
+  
+  verify(orderService).cancel(123L);
+}
+```
+
+## Testing with Security
+
+```java
+@Test
+@WithMockUser(roles = "ADMIN")
+void adminShouldDeleteOrder() {
+  assertThat(mvc.delete().uri("/orders/1"))
+    .hasStatus(HttpStatus.NO_CONTENT);
+}
+
+@Test
+void anonymousUserShouldBeForbidden() {
+  assertThat(mvc.delete().uri("/orders/1"))
+    .hasStatus(HttpStatus.UNAUTHORIZED);
+}
+```
+
+## Multiple Controllers
+
+```java
+@WebMvcTest({OrderController.class, ProductController.class})
+class WebLayerTest {
+  // Tests multiple controllers in one slice
+}
+```
+
+## Excluding Auto-Configuration
+
+```java
+@WebMvcTest(OrderController.class)
+@AutoConfigureMockMvc(addFilters = false) // Skip security filters
+class OrderControllerWithoutSecurityTest {
+  // Tests without security filters
+}
+```
+
+## Key Points
+
+1. Always mock services with @MockitoBean
+2. Use MockMvcTester for AssertJ-style assertions
+3. Test HTTP semantics (status, headers, content-type)
+4. Verify service method calls when side effects matter
+5. Don't test business logic here - that's for unit tests
+6. Leverage Java 25 text blocks for JSON payloads