fix: Add referenceTaskIds handling in ProtoUtils.java

Author: fjuma

spec-grpc/src/main/java/io/a2a/grpc/utils/ProtoUtils.java

@@ -164,6 +164,9 @@ public static io.a2a.grpc.Message message(Message message) {
                 builder.addAllParts(message.getParts().stream().map(ToProto::part).collect(Collectors.toList()));
             }
             builder.setMetadata(struct(message.getMetadata()));
+            if (message.getReferenceTaskIds() != null && !message.getReferenceTaskIds().isEmpty()) {
+                builder.addAllReferenceTaskIds(message.getReferenceTaskIds());
+            }
             return builder.build();
         }
 
@@ -875,7 +878,7 @@ public static Message message(io.a2a.grpc.MessageOrBuilder message) {
                     message.getMessageId().isEmpty() ? null :  message.getMessageId(),
                     message.getContextId().isEmpty() ? null :  message.getContextId(),
                     message.getTaskId().isEmpty() ? null :  message.getTaskId(),
-                    null, // referenceTaskIds is not in grpc message
+                    message.getReferenceTaskIdsList().isEmpty() ? null :  message.getReferenceTaskIdsList(),
                     struct(message.getMetadata())
             );
         }