fix: resolve issue where ValueError is not raised if both request and…

… flattened param are set

tests/fragments/test_nested_messages.proto

@@ -0,0 +1,41 @@
+// Copyright (C) 2024  Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+syntax = "proto3";
+
+package google.fragment;
+
+import "google/protobuf/struct.proto";
+import "google/api/client.proto";
+
+service MyService {
+  option (google.api.default_host) = "my.example.com";
+
+  rpc MyMethod(MethodRequest) returns (MethodResponse) {
+    option (google.api.method_signature) = "some_message";
+  }
+}
+
+message AnotherMessage {};
+
+message SomeMessage{
+  AnotherMessage another_message = 1;
+}
+
+message MethodRequest {
+  SomeMessage some_message = 1;
+}
+
+message MethodResponse {
+  google.protobuf.Value result = 1;
+}