InvalidOperationException when 'UseStringProtoEnumValueNames' enabled

[Havret]

Describe the bug

When I try to serialize an object created based on the following proto file

syntax = "proto3";

option csharp_namespace = "System.Text.Json.Protobuf.Tests";

message MessageWithEnum {
  TestEnum enum_field = 1;
  int32 regular_field = 2;
}

enum TestEnum {
  UNKNOWN = 0;
  FIRST_OPTION = 1;
  SECOND_OPTION = 2;
}

I'm getting the following exception:

System.InvalidOperationException
EnumType is only valid for enum fields.
   at Google.Protobuf.Reflection.FieldDescriptor.get_EnumType()
   at Protobuf.System.Text.Json.ProtobufConverter`1.<>c__DisplayClass3_0.<.ctor>b__2(FieldDescriptor fieldDescriptor) in /Users/havret/git/System.Text.Json.Protobuf/src/Protobuf.System.Text.Json/ProtobufConverter.cs:line 32
   at System.Linq.Enumerable.SelectIListIterator`2.ToArray()
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at Protobuf.System.Text.Json.ProtobufConverter`1..ctor(JsonSerializerOptions jsonSerializerOptions, JsonProtobufSerializerOptions jsonProtobufSerializerOptions) in /Users/havret/git/System.Text.Json.Protobuf/src/P

Expected behavior

No exception should be thrown.

Environment:

• OS: macOS Ventura 13.4.1 (22F82)
• Version: 1.2.0

Additional context

Serialization works fine when there are only enum fields.