Optional fields redeclared in canonical_creative lose the never-null marker, so nested dumps emit null (follow-up to #1587) #1620

Follow-up to #1587 / #1591.

Observed

Optional fields that adcp.types.canonical_creative redeclares don't carry the never-null marker (_RefuseJsonNull) that their generated counterparts do. They aren't omitted when unset in a nested dump, so a plain-pydantic parent emits null for them. Their schemas don't admit null.

from pydantic import BaseModel

from adcp.types import Product


class Envelope(BaseModel):
    products: list[Product]


product = Product.model_construct(product_id="p", name="n", description="d")
print("direct dump nulls:", sorted(k for k, v in product.model_dump(mode="json").items() if v is None))
body = Envelope.model_construct(products=[product]).model_dump(mode="json")
print("nested dump nulls:", sorted(k for k, v in body["products"][0].items() if v is None))

direct dump nulls: []
nested dump nulls: ['placements']

Optional fields checked on 9.1.0 with no marker (compare Product.channels, which has one):

• Product.placements
• Placement.format_options
• CreateMediaBuyRequest.packages
• UpdateMediaBuyRequest.packages and new_packages
• GetProductsResponse.products
• GetProductsRequest.filters
• ListCreativesRequest.filters

Expected

Redeclared optional fields keep the generated field's never-null metadata, so #1591's guarantee ("never-null fields omit None from every serializer") also holds for the canonical-creative types.

Impact

The Embedded Sales Agent keeps its own Product serializer that strips these nulls, because its wire responses nest SDK products in its own models.