Support consumer-configurable extra policy on AdCPBaseModel

[bokelley]

Summary

Consumers who subclass library types (e.g., Product, Package) need to control Pydantic's extra field policy per-environment:

• Development/CI: extra="forbid" to catch bugs
• Production: extra="ignore" for forward compatibility

Currently, every generated type inherits model_config = ConfigDict(extra='allow') from the schemas' additionalProperties: true. Consumers must manually override model_config on every subclass — @KonstantinMirin reports doing this 28 times across 4 schema files in prebid/salesagent.

Proposed Solution

Add a __init_subclass__ hook to AdCPBaseModel that respects a class-level _adcp_extra_mode attribute:

class AdCPBaseModel(BaseModel):
    @classmethod
    def __init_subclass__(cls, **kwargs: Any) -> None:
        super().__init_subclass__(**kwargs)
        extra_mode = getattr(cls, '_adcp_extra_mode', None)
        if extra_mode is not None and 'extra' not in cls.model_config:
            cls.model_config = ConfigDict(extra=extra_mode)

Consumers set it once on their base:

class SalesAgentBaseModel(AdCPBaseModel):
    _adcp_extra_mode = "forbid"  # or "ignore" in prod

class Product(LibraryProduct):
    pass  # inherits "forbid" automatically

Why not an env var?

An env var (ADCP_EXTRA_MODE) is simpler but:

• Global — affects all types in the process, including library internals
• Can't scope different policies to different type hierarchies
• Requires process restart to change modes

The __init_subclass__ hook is scoped to consumer subclasses and composable.

Origin

Raised by @KonstantinMirin in #150.

[bokelley]

After running this by our Python/Pydantic expert and product architect, the __init_subclass__ proposal has fundamental problems:

Why __init_subclass__ won't work

1. Timing — fires after Pydantic's metaclass has already built the schema from model_config. Mutating config at that point doesn't change validation behavior.
2. Guard condition bug — 'extra' not in cls.model_config is always True because config is inherited from the parent (which has extra='allow'). The condition never fires.
3. model_rebuild() interaction — _ergonomic.py calls model_rebuild(force=True) at import time, creating fragile order-dependent behavior.

Why the env var won't work either

• Evaluates once at import time — setting it after import does nothing
• Global — would break types that genuinely need extra='allow' (ExtensionObject, ContextObject)
• Environment variables for type system behavior is invisible to code readers

Root cause

The real issue is --allow-extra-fields in scripts/generate_models.py, which forces extra='allow' on every generated type regardless of what the schemas actually specify. Some types have additionalProperties: true (extension points) and should allow extras. Others don't and should forbid them.

Proposed fix

1. Remove --allow-extra-fields from code generation
2. Let datamodel-codegen respect each schema's actual additionalProperties value
3. Types with additionalProperties: true get allow, others get forbid
4. No new API surface needed

This is a bigger change than the original proposal (affects all generated types), so it needs careful analysis of which schemas specify what. But it's the correct fix — it makes the Python types match the spec rather than papering over a blanket override with configuration hooks.

@KonstantinMirin would this work for your use case? If the library types matched the spec's additionalProperties values, you'd get forbid on strict types by default and only need to override the few extension-point types if you wanted ignore in prod.