Use field names for serializing protos.

For backward compatibility, the parsing works with both field names and numbers.

PiperOrigin-RevId: 552781329

fuzztest/internal/domains/protobuf_domain_impl.h

diff --git a/fuzztest/internal/domains/protobuf_domain_impl.h b/fuzztest/internal/domains/protobuf_domain_impl.h
index 785a9db..ceae061 100644
--- a/fuzztest/internal/domains/protobuf_domain_impl.h
+++ b/fuzztest/internal/domains/protobuf_domain_impl.h
@@ -825,16 +825,20 @@
     for (const auto& sub : *subs) {
       auto pair_subs = sub.Subs();
       if (!pair_subs || pair_subs->size() != 2) return std::nullopt;
-      auto number = (*pair_subs)[0].GetScalar<int>();
-      if (!number) return std::nullopt;
-      auto* field = GetField(*number);
+      FieldDescriptor* field = nullptr;
+      if (auto number = (*pair_subs)[0].GetScalar<int>(); number.has_value()) {
+        field = GetField(*number);
+      } else if (auto name = (*pair_subs)[0].GetScalar<std::string>();
+                 name.has_value()) {
+        field = GetField(*name);
+      }
       if (!field) return std::nullopt;
       present_fields.insert(field->number());
       std::optional<GenericDomainCorpusType> inner_parsed;
       VisitProtobufField(field,
                          ParseVisitor{*this, (*pair_subs)[1], inner_parsed});
       if (!inner_parsed) return std::nullopt;
-      out[*number] = *std::move(inner_parsed);
+      out[field->number()] = *std::move(inner_parsed);
     }
     for (const FieldDescriptor* field :
          GetProtobufFields(prototype_.Get()->GetDescriptor())) {
@@ -875,7 +879,7 @@
       FUZZTEST_INTERNAL_CHECK(field, "Field not found by number: ", number);
       IRObject& pair = subs.emplace_back();
       auto& pair_subs = pair.MutableSubs();
-      pair_subs.emplace_back(number);
+      pair_subs.emplace_back(GetFieldName(field));
       VisitProtobufField(
           field, SerializeVisitor{*this, inner, pair_subs.emplace_back()});
     }