[http-server-csharp] Placeholder is generated for long-running operation

[ArcturusZhang]

I am trying this server generator on the contoso project: https://github.com/Azure/azure-rest-api-specs/blob/main/specification/contosowidgetmanager/Contoso.WidgetManager/main.tsp#L50

The two LRO generate with placeholders:

        ///<summary>
        /// Creates or updates a Widget asynchronously.
        ///</summary>
        [HttpPatch]
        [Route("/widgets/{widgetName}")]
        [ProducesResponseType((int)HttpStatusCode.Created, Type = typeof(WidgetSuite))]
        [ProducesResponseType((int)HttpStatusCode.OK, Type = typeof(WidgetSuite))]
        public virtual async Task<IActionResult> CreateOrUpdateWidget([FromQuery(Name = "api-version")] string apiVersion, string widgetName, [FromHeader(Name = "Content-Type")] string contentType = "application/merge-patch+json", WidgetSuite body)
        {
            var result = await WidgetsImpl.CreateOrUpdateWidgetAsync(apiVersion, widgetName, contentType, body);
            return Ok(result);
        }