dartgen: materialize mapped class lists in toJson to avoid jsonEncode failures

[Copilot]

goctl api dartgen was emitting toJson() for class-list fields with map(...) only, producing MappedListIterable values that fail jsonEncode. This change updates generation so list fields serialize as concrete JSON arrays.

• Generator fix (legacy + v2 templates)

  • Updated Dart toJson() generation for isClassListType members to append .toList().
  • Ensures generated payloads are List-backed and encodable by dart:convert.

• Regression coverage

  • Added/updated focused dartgen test assertions to verify generated toJson() for class-list fields includes .toList() and does not regress.

Example of generated output after this change:

Map<String, dynamic> toJson() {
  return {
    'items': items.map((i) => i?.toJson()).toList(),
  };
}