Support nested @body

[nguerrera]

Fix #1141

This was originally intended to be supported and got broken during development when we added automatic body for unannotated properties at the same time. This is why the error message is hard to understand. This makes it work. The issue is still tagged design, but I consider it a bug fix.

[github-actions]

Changes in this PR will be published to the following url to try(check status of TypeSpec Pull Request Try It pipeline for publish status):
Playground: https://cadlplayground.z22.web.core.windows.net/prs/2029/

Website: https://cadlwebsite.z1.web.core.windows.net/prs/2029/

[timotheeguerin]

I see it ignores other properties in the nested model, is it expected
https://cadlplayground.z22.web.core.windows.net/prs/2029/?c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7Cgp1c2luZyBUeXBlU3BlYy5IdHRwOwoKLy8gQHJvdXRlKCJpbXBsaWNpdCIpIG9wIMgOKGE6IHN0cmluZywgYsgLKTogdm9pZM5AZmFpbC0xxj7EDDHMO0Bib2R530HFQTLKQTIoxB4gIMpHxhBiOiB7xgogx1hjyCI7yBhkzxJ9xAfKds1zM8pzMygKx2DVXc9axFfJVA%3D%3D

in the playground you can uncomment the other fail operation and those fail as expected but not if you have

 op fail3(
  b: {
    @body c: string;
    d: string;
  }
): void;

[nguerrera]

Hmm, FWIW, I think @body has had the impact already of dropping other unannotated properties on the response.

https://cadlplayground.z22.web.core.windows.net/?c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7Cgp1c2luZyBUeXBlU3BlYy5IdHRwOwoKQHJvdXRlKCJ0ZXN0IikKb3AgxAooKTogewogIGE6IHN0cmluZzsKICBAYm9keSBiyhN9Ow%3D%3D

We only consider top-level parameters unannotated. We could perhaps make this an error, but it feels like then we should do the same on response.

[timotheeguerin]

hhm yeah feels like the response here should be consistent

[timotheeguerin]

I think this should be an error (or at least a warning) some of the data is being lost here

[nguerrera]

Trying to figure out how far back that response behavior goes, I think to the inception of @body

[timotheeguerin]

This already being inconsistent maybe we don't need to do that in this PR though and can file an issue about it.

[nguerrera]

#2075

[timotheeguerin]

This looked good I think just need a new issue to talk about what should we do if there is extra unannoted properties and @body for the nested case and in response