Incorrect codegen for polymorphic DU "base" type #852
Description
For polymorphic DU base types, we emit an inline struct definition for the serializer impl. This happens here.
It works for simple model field types. However, if a field requires custom serialization (e.g. base64 encoded bytes, OffsetDateTime), then the codegen is incorrect as the serialization helpers will be absent.
What we need to do is leverage the model field builder code here to emit this type.