[BUG] dart-dio + polymorphism + json_serializable fails to produce polymorphic classes

Using the schema from: https://github.com/OpenAPITools/openapi-generator/issues/21297 fails to produce polymorphic classes with

generate -i swagger.yaml -g dart-dio -o ./_swagger --additional-properties=sourceFolder=openapi/src,pubLibrary=flutter_common.openapi,pubName=flutter_common,serializationLibrary=json_serializable"

Produces:

@JsonSerializable(...)
class Dog { }

Should be:

@JsonSerializable(...)
class Dog extends Pet { }       // extends

Without json_serializable, Dog extends Pet is produced, but BuiltValue classes cause a variety of problems (they're immutable and have non-standard constructors making them not play nice with other code generators that expect plain old Dart objects, or legacy projects using -g dart and wanting add polymorphism, (which is not support with -g dart).

Would be happy to work on this if there's a guide to implementing generators.

Start with the dart-dio generator and its json_serializable option, using the schema referenced in issue 21297 to compare the generated Dog and Pet classes. Done means json_serializable output preserves the polymorphic relationship so Dog extends Pet, without switching to BuiltValue.