Generated FieldMask class does not serialize correctly

[nicjohnson145]

Summary

FieldMask json/dict serializes as list rather than comma separated string

Reproduction Steps

Attached tarball with simple project reproducing the issue,

field_mask_repro.tar.gz

but in short:

• Define a message with a google FieldMask field

syntax = 'proto3';

package foo.v1;

import "google/protobuf/field_mask.proto";

message Bar {
  google.protobuf.FieldMask mask = 1;
}

• construct an instance of said message and call to_dict/to_json on it

Expected Results

the FieldMask portion serializes as a comma separate string per the doc string (and other langauge's implementations)

    # JSON Encoding of Field Masks

    In JSON, a field mask is encoded as a single string where paths are
    separated by a comma. Fields name in each path are converted
    to/from lower-camel naming conventions.

    As an example, consider the following message declarations:

        message Profile {
          User user = 1;
          Photo photo = 2;
        }
        message User {
          string display_name = 1;
          string address = 2;
        }

    In proto a field mask for `Profile` may look as such:

        mask {
          paths: "user.display_name"
          paths: "photo"
        }

    In JSON, the same mask is represented as below:

        {
          mask: "user.displayName,photo"
        }

Actual Results

it serializes as a list of strings

❯ uv run bar
      Built bar @ file:///home/policyfly/tmp/field_mask_repro/bar
      Built foo @ file:///home/policyfly/tmp/field_mask_repro/gen/python
Uninstalled 2 packages in 0.47ms
Installed 2 packages in 1ms
{'mask': {'paths': ['a.b', 'a.c']}}
{"mask": {"paths": ["a.b", "a.c"]}}

System Information

❯ uv run python --version
Python 3.12.7

not running with protoc, but running with buf & uv

❯ buf --version
1.50.0

❯ uv self version
uv 0.9.28

Checklist

• I have searched the issues for duplicates.
• I have shown the entire traceback, if possible.
• I have verified this issue occurs on the latest prelease of betterproto which can be installed using pip install -U --pre betterproto, if possible.