[http-server-csharp] generator throws error when void is the return type in operation

[ArcturusZhang]

if I have this typespec:

    @post
    @route("/hello")
    op hello(): void;

and I run the generator, it gives me this error:

../<unknown location>:1:1 - error @typespec/http-server-csharp/invalid-intrinsic: Intrinsic type void is not valid in this context.  Please use a model, enum, union, scalar, or the unknown type

there are multiple issues in this error message.

1. the location of the error is not clearly flagged. Lucky for me, my typespec only have one void therefore I am able to locate the issue.
2. the reason does not make any sense because for a post, it is very common that people would use void as its response type indicating that this operation returns nothing in body.