Support discriminator where discriminated value is a union

[JoshLove-msft]

A model like this will cause errors when generating because the discriminator property can have multiple possible values. We should add support for this. We should also request a Spector scenario for it.

@discriminator("type")
model ResponsesInputContent extends ResponsesContent {
  type:
    | ResponsesContentType.input_text
    | ResponsesContentType.input_audio
    | ResponsesContentType.input_image
    | ResponsesContentType.input_file;
}