Use serializationOptions to determine request/response format

Observed in https://github.com/microsoft/typespec/blob/main/packages/http-specs/specs/type/model/visibility/main.tsp#L51-L67

Since this is a `GET` there shouldn't be a body, which is presumably why `serializationOptions` is `undefined`. However, we set this type as the body. In addition, note that the model contains a query param which is currently omitted in codegen.