fix: Use JsonBaseModel in FunctionTool (re. #473)

Author: vorburger

core/src/main/java/com/google/adk/tools/FunctionTool.java

@@ -18,7 +18,7 @@
 
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.ObjectMapper;
-import com.fasterxml.jackson.datatype.jdk8.Jdk8Module;
+import com.google.adk.JsonBaseModel;
 import com.google.adk.agents.InvocationContext;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
@@ -42,9 +42,9 @@
 
 /** FunctionTool implements a customized function calling tool. */
 public class FunctionTool extends BaseTool {
-  private static final ObjectMapper OBJECT_MAPPER =
-      new ObjectMapper().registerModule(new Jdk8Module());
+
   private static final Logger logger = LoggerFactory.getLogger(FunctionTool.class);
+  private static final ObjectMapper OBJECT_MAPPER = JsonBaseModel.getMapper();
 
   @Nullable private final Object instance;
   private final Method func;