Expose `OutputObjectDefinition` in `output.py` instead of private `_output.py`

[moritzwilksch]

Description

I want to be able to use native structured generation in the direct mode. Currently, this works but requires to import from a private module. This would be cleaner if users could from pydantic_ai.output import OutputObjectDefinition.

All other possible argument types of ModelRequestParameters are available in public modules, this is the only one missing.

from pydantic import BaseModel
from pydantic_ai._output import OutputObjectDefinition  # <<<<<<<< private import
from pydantic_ai.direct import model_request_sync
from pydantic_ai.messages import (
    ModelRequest,
    SystemPromptPart,
    UserPromptPart,
)
from pydantic_ai.models import ModelRequestParameters
from pydantic_ai.models.openai import (
    OpenAIResponsesModel,
)

model = OpenAIResponsesModel(
    model_name="gpt-5-mini",
)


class LineItem(BaseModel):
    item_name: str
    quantity: int


messages = [
    ModelRequest(
        [
            SystemPromptPart("Extract the line items from the message."),
            UserPromptPart("hi i want to order five pizzas."),
        ]
    )
]

request_parameters = ModelRequestParameters(
    output_mode="native",
    output_object=OutputObjectDefinition(
        json_schema=LineItem.model_json_schema(),
    ),
)

response = model_request_sync(
    model,
    messages,
    model_request_parameters=request_parameters,
)
print(response)
# ModelResponse(
#     parts=[
#         ThinkingPart(
#             content="",
#             id="...",
#             signature="...",
#             provider_name="openai",
#         ),
#         TextPart(
#             content='{"item_name":"pizza","quantity":5}',
#             id="...",
#         ),
#     ],
#     usage=RequestUsage(
#         input_tokens=56, output_tokens=148, details={"reasoning_tokens": 128}
#     ),
#     model_name="gpt-5-mini-2025-08-07",
#     timestamp=datetime.datetime(2025, 9, 23, 14, 54, 25, tzinfo=TzInfo(UTC)),
#     provider_name="openai",
#     provider_details={"finish_reason": "completed"},
#     provider_response_id="...",
#     finish_reason="stop",
# )

References

No response