Map Field(json_schema_extra) to extend:name / extend:descriptions

[jordanalexmeyer]

Summary

Python parity for extend-hq/extend-typescript-sdk#90 (zod .meta() support). pydantic_to_extend_schema now reads Field(json_schema_extra={...}):

• extend:name (string) is emitted on every node type that carries a description: primitives, enums, dates, arrays, nested objects, and ExtendCurrency / ExtendSignature / ExtendDate
• extend:descriptions (list of strings, one per enum value) is emitted on enum outputs — both Literal[...] and string Enum fields
• Unrelated json_schema_extra keys and wrong-typed values are silently ignored, matching the TypeScript behavior
• Works under pydantic v1 and v2 (v1 stashes unknown Field() kwargs in FieldInfo.extra, which is handled)

class Schema(pydantic.BaseModel):
    status: Optional[Literal["active", "inactive"]] = pydantic.Field(
        None,
        json_schema_extra={
            "extend:name": "AccountStatus",
            "extend:descriptions": ["Account is active", "Account is inactive"],
        },
    )

Test plan

• 11 new tests mirroring the TypeScript PR's coverage, plus wrong-type / ignored-key / non-enum cases
• Full suite passes (270 passed, 1 skipped), ruff clean
• Schema conversion tests also run under pydantic 1.10.18 — all new tests pass (one pre-existing recursion test failure there reproduces on unmodified master)