Use `std::string` to convert `std::string_view` to `absl::string_view` when they are different.

PiperOrigin-RevId: 552833065

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 ceae061..f6c1249 100644
--- a/fuzztest/internal/domains/protobuf_domain_impl.h
+++ b/fuzztest/internal/domains/protobuf_domain_impl.h
@@ -830,7 +830,7 @@
         field = GetField(*number);
       } else if (auto name = (*pair_subs)[0].GetScalar<std::string>();
                  name.has_value()) {
-        field = GetField(*name);
+        field = GetField(std::string(*name));
       }
       if (!field) return std::nullopt;
       present_fields.insert(field->number());