fix(pydantic): recurse into oneOf variants in strict JSON schema

[alvinttang]

Summary

_ensure_strict_json_schema() in src/openai/lib/_pydantic.py recurses into anyOf and allOf variants but skips oneOf. Pydantic v2 generates oneOf (not anyOf) for discriminated unions — i.e. fields using discriminator=...:

class Cat(BaseModel):
    pet_type: Literal["cat"]
    meows: int

class Dog(BaseModel):
    pet_type: Literal["dog"]
    barks: float

class PetOwner(BaseModel):
    name: str
    pet: Union[Cat, Dog] = Field(discriminator="pet_type")

This produces a schema with "oneOf": [{"$ref": "#/$defs/Cat"}, {"$ref": "#/$defs/Dog"}] in the pet property. Any inline object schemas nested inside oneOf (rather than via $ref) would not receive additionalProperties: false or the required array, causing the OpenAI API to reject the schema with:

Invalid schema for response_format: 'additionalProperties' is required to be supplied and to be false.

Fix

Add oneOf handling that mirrors the existing anyOf logic (7 lines).

Test plan

• Added test_discriminated_union_oneOf regression test
• Verified anyOf behavior is unchanged
• Verified $defs inside discriminated unions still get strict treatment

🤖 Generated with Claude Code