Fix missing resultType on complete result responses (#1684)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Tarek Mahmoud Sayed <tarekms@ntdev.microsoft.com>

src/ModelContextProtocol.Core/Protocol/EmptyResult.cs

@@ -9,5 +9,5 @@ namespace ModelContextProtocol.Protocol;
 public sealed class EmptyResult : Result
 {
     [JsonIgnore]
-    internal static EmptyResult Instance { get; } = new();
+    internal static EmptyResult Instance { get; } = new() { ResultType = "complete" };
 }