Fix C# completion resolve in VS (#11905)

Fixes the "Assert" that Phil was seeing

Author: davidwengier

src/Razor/src/Microsoft.CodeAnalysis.Remote.Razor/Completion/RemoteCompletionService.cs

@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
+using System.Text.Json;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Razor;
@@ -312,6 +313,13 @@ private async ValueTask<VSInternalCompletionItem> ResolveCSharpCompletionItemAsy
     {
         request.Data = DelegatedCompletionHelper.GetOriginalCompletionItemData(request, containingCompletionList, resolutionContext.OriginalCompletionListData);
 
+        // Roslyn expects data to be JsonElement because we're calling into their LSP handler, but we cache their data as its underlying object, so lets
+        // make sure to serialize if we need to.
+        if (request.Data is not JsonElement)
+        {
+            request.Data = JsonSerializer.SerializeToElement(request.Data, JsonHelpers.JsonSerializerOptions);
+        }
+
         var documentSnapshot = context.Snapshot;
         var generatedDocument = await documentSnapshot.GetGeneratedDocumentAsync(cancellationToken).ConfigureAwait(false);