[v0ropaev]

Generated JSON only treats google.protobuf.NullValue specially for oneof members and extensions. Every other field shape goes through the generic open-enum helpers, so an optional, required, repeated or map NullValue field serializes as the string "NULL_VALUE", in owned and view serialization alike. On the way in, opt_enum reads {"opt": null} as an unset field, so the value is dropped and also disappears from the binary encoding.

struct.proto says the JSON representation of NullValue is JSON null, and protobuf-go and the C++ parser read null as a present NULL_VALUE. buffa's own DynamicMessage already does both, so right now generated types and DynamicMessage produce different JSON for the same bytes.

message M {
  optional google.protobuf.NullValue opt = 1;
  repeated google.protobuf.NullValue list = 2;
  map<string, google.protobuf.NullValue> m = 3;
}

before: {"opt":"NULL_VALUE","list":["NULL_VALUE"],"m":{"k":"NULL_VALUE"}}
after:  {"opt":null,"list":[null],"m":{"k":null}}

and serde_json::from_str::<M>(r#"{"opt":null}"#)?.opt was None, now it's Some(NULL_VALUE).

Codegen now sends NullValue fields to dedicated json_helpers modules (null_value, opt_null_value, repeated_null_value, map_null_value, and bytes_key_null_value_map for the bytes keys that strict_utf8_mapping produces). They write null, accept null, "NULL_VALUE" or a number, and keep the ignore_unknown_enum_values filtering the enum helpers have. An unknown number in a NullValue field is also written as null, the same as protobuf-go, C++ and DynamicMessage. The generated view Serialize writes null for the same shapes.

The new modules deliberately don't share code with repeated_enum or map_enum, so #766, which starts rejecting null elements in those, won't affect NullValue lists and maps. I checked that the two apply together and both sets of tests pass.

The tests build proto3, edition 2023 and proto2 fixtures with views and JSON. They check owned and view output for the implicit, optional, required, repeated and map shapes (string, integer, bool and bytes keys), and parsing for the explicit-presence and container shapes. Generated code from this version needs the matching buffa, since it names the new helper modules.

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