[http-server-csharp] inconsistent status code in some generated operations

[ArcturusZhang]

when we have this spec:

@delete op delete(
    @path id: TodoItem.id,
  ): WithStandardErrors<NoContentResponse | NotFoundErrorResponse>;

this code was generated:

        [HttpDelete]
        [Route("/items/{id}")]
        [ProducesResponseType((int)HttpStatusCode.NoContent, Type = typeof(void))]
        public virtual async Task<IActionResult> Delete(long id)
        {
            await TodoItemsOperationsImpl.DeleteAsync(id);
            return Ok();
        }

This operation (when succeeded) should always produce 204, but we are returning Ok() which represents to 200.

[lirenhe]

Update the label as this is a server gen issue.