fix: Address SonarQube issues in CRaC implementation

- Add proper assertions to TracingUtilsCracTest to fix Blocker issue
- Add null check for objectMapper in beforeCheckpoint to fix Critical issue
- Maintain singleton pattern for CRaC Resource registration (Info issue acceptable)

All tests passing: 28 tests, 0 failures

Author: dcabib

powertools-tracing/src/main/java/software/amazon/lambda/powertools/tracing/TracingUtils.java

@@ -235,7 +235,9 @@ public void beforeCheckpoint(Context<? extends Resource> context) throws Excepti
         serviceName();
 
         // Initialize ObjectMapper for JSON serialization
-        objectMapper.writeValueAsString("dummy");
+        if (objectMapper != null) {
+            objectMapper.writeValueAsString("dummy");
+        }
     }
 
     @Override

powertools-tracing/src/test/java/software/amazon/lambda/powertools/tracing/TracingUtilsCracTest.java

@@ -34,7 +34,9 @@ void testPrimeMethodDoesNotThrowException() {
     @Test
     void testTracingUtilsLoadsSuccessfully() {
         // Simply calling TracingUtils.prime() should trigger CRaC registration
-        TracingUtils.prime();
-        // If we get here without exception, the test passes
+        assertThatNoException().isThrownBy(() -> TracingUtils.prime());
+        
+        // Verify that TracingUtils class is loaded and accessible
+        assertThatNoException().isThrownBy(() -> TracingUtils.objectMapper());
     }
 }