Discriminated unions seem broken for JS http client

https://typespec.io/docs/standard-library/discriminated-types/#inject-discriminator-inline

It appears like discriminated unions don't work with the newer form with generating the js http client. Possibly related to https://github.com/microsoft/typespec/pull/6286/files?

[Playground Link](https://typespec.io/playground/?options=%7B%7D&sample=Discriminated+unions)

This also causes issues.
```
@discriminated(#{envelope: "none"})
union Widget {
heavy: WidgetBase,
light: WidgetBase,
}
```