Unnamed union with named members on the model level

### Clear and concise description of the problem

Problem as stated in https://github.com/microsoft/typespec/discussions/7124

Scenario: model foo has a property bar and one of 2 properties baz and fox.

Potential modelling (or any equivalent that would allow anonymous reference to baz_or_fox where the union properties are named but are directly placed in foo):
```
model foo {
bar: uint32;
...baz_or_fox;
}

union baz_or_fox {
baz: uint32;
fox: uint32;
}
```

Currently it throws "Cannot spread properties of non-model type.".

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.