[BUG][python-fastapi] Generated server model `to_dict()` excludes `readOnly` fields, dropping valid response properties

OpenAPI Generator version

7.21.0

OpenAPI declaration file content or url

components:
  schemas:
    Process:
      type: object
        .....
        completionProgress:
          type: number
          format: double
          readOnly: true
          minimum: 0
          maximum: 1
          example: 0.75
          multipleOf: 0.0001
          description: "Completion progress of the quiz (0 to 1). Computed by backend."
      additionalProperties: false

What I expected

Since python-fastapi is a server generator, generated response serialization should not exclude readOnly properties. For OpenAPI, readOnly means the field may appear in responses and should not be sent in requests. So a server response serializer should keep fields like completionProgress.

Expected generated code shape:

    def to_dict(self) -> Dict[str, Any]:
        """Return the dictionary representation of the model using alias.

        This has the following differences from calling pydantic's
        `self.model_dump(by_alias=True)`:

        * `None` is only added to the output dict for nullable fields that
          were set at model initialization. Other fields with value `None`
          are ignored.
        """
        _dict = self.model_dump(
            by_alias=True,
            exclude={
            },
            exclude_none=True,
        )
        return _dict

What it returns

The generated server model excludes readOnly fields unconditionally in to_dict().

Generated code currently looks like this:

    def to_dict(self) -> Dict[str, Any]:
        """Return the dictionary representation of the model using alias.

        This has the following differences from calling pydantic's
        `self.model_dump(by_alias=True)`:

        * `None` is only added to the output dict for nullable fields that
          were set at model initialization. Other fields with value `None`
          are ignored.
        * OpenAPI `readOnly` fields are excluded.
        """
        excluded_fields: Set[str] = set([
            "completion_progress",
        ])

        _dict = self.model_dump(
            by_alias=True,
            exclude=excluded_fields,
            exclude_none=True,
        )
        return _dict

As a result, valid response-only fields silently disappear from server responses.
For example, if the server model instance has:

Process(
    ....
    completion_progress=0.75,
)

then to_dict() produces a payload without completion_progress, even though it is a valid response field.

python-fastapi is a server generator, so unconditionally excluding readOnly fields from to_dict() appears to conflict with the OpenAPI meaning of readOnly.

This also seems closely related to the broader request/response modeling problem discussed in #4190.

Suggested fix

For python-fastapi server generation, to_dict() should not exclude readOnly fields by default.

Possible approaches:

• Generate server to_dict() without excluding readOnly fields.
• Split request vs response serialization helpers (for example to_request_dict() vs to_dict() / to_response_dict()).
• Generate distinct request and response models/serialization paths for schemas using readOnly / writeOnly.

Start with the python-fastapi model-generation entry point responsible for the generated to_dict() method, and compare its readOnly handling with the expected output shown here. Verify that completionProgress remains in server response serialization, then add or run a regression test showing that a populated readOnly field is preserved.