Support discriminator where discriminated value is a union

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;
}

```