AdditionalProperties property type should be based on the visibility rather than the usage

If an InputModel is only used for Output, all of its properties should be readonly.

The below anonymous model is only used for Output
https://github.com/microsoft/typespec/blob/fdefc060e7ee20ccee94143872baa1d7dd50176c/docs/samples/client/csharp/SampleService/main.tsp#L406C4-L411
```
op ListWithNextLink(): {
@pageItems
things: Thing[];

@nextLink next?: url;
};
```
The usage is correct in [tspCodeModel.json](https://github.com/microsoft/typespec/blob/fdefc060e7ee20ccee94143872baa1d7dd50176c/packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/tspCodeModel.json#L2796-L2799)

But the generated SDK is not readonly, it should be IReadOnlyList
https://github.com/microsoft/typespec/blob/fdefc060e7ee20ccee94143872baa1d7dd50176c/docs/samples/client/csharp/SampleService/SampleClient/src/Generated/Models/ListWithNextLinkResponse.cs#L36