[http-server-csharp] operations with normal response and error response is generated to return JsonNode

[ArcturusZhang]

In todoApp, we have this operation:

  @get op get(@path id: TodoItem.id): TodoItem | NotFoundResponse;

but the generated operation for this is:

    public interface ITodoItemsOperations
    {
        Task<JsonNode> GetAsync(long id);
    }

which is not a really good API.
Other operations are returning the WithStandardErrors<T | NotFoundResponse> but they work fine.

[markcowl]

This is about rendering of unions, which we are handlign with a separate issue: #5179