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/context-caching.md

@@ -0,0 +1,115 @@
+# Context Caching
+
+Optimize Spring Boot test suite performance through context caching.
+
+## How Context Caching Works
+
+Spring's TestContext Framework caches application contexts based on their configuration "key". Tests with identical configurations reuse the same context.
+
+### What Affects the Cache Key
+
+- @ContextConfiguration
+- @TestPropertySource
+- @ActiveProfiles
+- @WebAppConfiguration
+- @MockitoBean definitions
+- @TestConfiguration imports
+
+## Cache Key Examples
+
+### Same Key (Context Reused)
+
+```java
+@WebMvcTest(OrderController.class)
+class OrderControllerTest1 {
+  @MockitoBean private OrderService orderService;
+}
+
+@WebMvcTest(OrderController.class)
+class OrderControllerTest2 {
+  @MockitoBean private OrderService orderService;
+}
+// Same context reused
+```
+
+### Different Key (New Context)
+
+```java
+@WebMvcTest(OrderController.class)
+@ActiveProfiles("test")
+class OrderControllerTest1 { }
+
+@WebMvcTest(OrderController.class)
+@ActiveProfiles("integration")
+class OrderControllerTest2 { }
+// Different contexts loaded
+```
+
+## Viewing Cache Statistics
+
+### Spring Boot Actuator
+
+```yaml
+management:
+  endpoints:
+    web:
+      exposure:
+        include: metrics
+```
+
+Access: `GET /actuator/metrics/spring.test.context.cache`
+
+### Debug Logging
+
+```properties
+logging.level.org.springframework.test.context.cache=DEBUG
+```
+
+## Optimizing Cache Hit Rate
+
+### Group Tests by Configuration
+
+```
+ tests/
+   unit/           # No context
+   web/            # @WebMvcTest
+   repository/     # @DataJpaTest  
+   integration/    # @SpringBootTest
+```
+
+### Minimize @TestPropertySource Variations
+
+**Bad (multiple contexts):**
+
+```java
+@TestPropertySource(properties = "app.feature-x=true")
+class FeatureXTest { }
+
+@TestPropertySource(properties = "app.feature-y=true")
+class FeatureYTest { }
+```
+
+**Better (grouped):**
+
+```java
+@TestPropertySource(properties = {"app.feature-x=true", "app.feature-y=true"})
+class FeaturesTest { }
+```
+
+### Use @DirtiesContext Sparingly
+
+Only when context state truly changes:
+
+```java
+@Test
+@DirtiesContext // Forces context rebuild after test
+void testThatModifiesBeanDefinitions() { }
+```
+
+## Best Practices
+
+1. **Group by configuration** - Keep tests with same config together
+2. **Limit property variations** - Use profiles over individual properties
+3. **Avoid @DirtiesContext** - Prefer test data cleanup
+4. **Use narrow slices** - @WebMvcTest vs @SpringBootTest
+5. **Monitor cache hits** - Enable debug logging occasionally