Canonical response fields typed SchemaVariant lose element types (pyright errors, Any under the mypy plugin) #1416

Since #1392 made the canonical models real subclasses, src/adcp/types/canonical_creative.py annotates 10 overriding fields as SchemaVariant[...] (lines ~692, 701, 704, 845, 885, 917, 930, 936, 1000, 1023). These include GetProductsResponse.products, CreateMediaBuyResponse1.packages, ListCreativesResponse.creatives and Product.format_options. The deleted canonical_creative.pyi typed them precisely (e.g. list[Product] | None).

SchemaVariant was designed for adopter subclass overrides. With adcp.types.mypy_plugin it becomes Any; without it, it isn't a valid type at all. So for library-internal overrides:

• pyright / Pylance: list[Product] is not assignable to SchemaVariant, and SchemaVariant is not iterable, on ordinary adopter code like for p in resp.products.
• mypy without the plugin: the same errors.
• mypy with the plugin: no errors, but the fields are Any, so element typing is silently lost.

CI misses it because the repo enables the plugin and doesn't gate pyright.

Expected: inside the library, declare these overrides with their precise types, using targeted suppressions for the intentional override (# type: ignore[assignment] / # pyright: ignore[reportIncompatibleVariableOverride]), consistent with scripts/check_type_ignore_contract.py. Adopters should see list[Product] | None etc. under both checkers, with or without the plugin.

Acceptance: a strict adopter fixture iterating each of the 10 fields and using an element attribute passes mypy --strict with and without the plugin, and pyright. Add pyright on tests/type_checks/ to CI, or at least to make typecheck-all, so this can't regress silently.

Found in the post-merge audit of #1392.