Suport ellipsis as a option for action inputs, or another way to specifiy "not set" which isn't "None"

When writing the Camera capture functionality for the OpenFlexure microscope we need to support multiple cameras. Writing some generic higher level camera operations as the BaseCamera Thing requires calling internal methods on each camera.

One arguments is wait, for the picamera wait of None has a meaning, our default is 0.9. I would like to be able to do:

    @lt.thing_action
    def capture_jpeg(
        self,
        metadata: lt.deps.GetThingStates,
        logger: lt.deps.InvocationLogger,
        stream_name: str = "main",
        wait: Optional[float | EllipsisType] = ...,
    ) -> JPEGBlob:
        """Acquire one image from the camera as a JPEG.

        This will use the internal capture image functionally of capture_image if
        the specific camera being used.

        :param metadata: LabThings GetThingStates dependency, automatically injected.
        :param logger: LabThings InvocationLogger dependency, automatically injected.
        :param stream_name: A stream name supported by this camera.
        :param wait: (Optional, float) Set a timeout in seconds. If not set it will
            use the default for the underlying camera.
        """
        fname = datetime.now().strftime("%Y-%m-%d-%H%M%S.jpeg")
        directory = tempfile.TemporaryDirectory()
        jpeg_path = os.path.join(directory.name, fname)

        # Using Ellipsis to specify no input specified. As `None` set for wait may
        # have a meaning as it does for the Picamera. If wait is Ellipsis then
        # do not specify.
        if wait is Ellipsis:
            img = self.capture_image(stream_name)
        else:
            img = self.capture_image(stream_name, wait)
        self._save_capture(
            jpeg_path=jpeg_path,
            image=img,
            metadata=metadata,
            logger=logger,
        )

        return JPEGBlob.from_temporary_directory(directory, fname)

However, I get the following Pydantic error due to the Ellipsis:

fastapi.exceptions.FastAPIError: Invalid args for response field! Hint: check that typing.Union[float, ellipsis, NoneType] is a valid Pydantic field type. If you are using a return type annotation that is not a valid Pydantic field (e.g. Union[Response, dic...

Start at the action input handling and the FastAPI/Pydantic validation path described by the error. Trace how annotated defaults and unions are converted into request fields; done means an action can distinguish an omitted value from an explicit None without triggering the reported validation error, while preserving existing input behavior.