Make RequestId non-required (#984)

src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

@@ -250,7 +250,6 @@ internal sealed class StreamableHttpHandler(
     {
         var jsonRpcError = new JsonRpcError
         {
-            Id = default,
             Error = new()
             {
                 Code = errorCode,

src/ModelContextProtocol.Core/Protocol/JsonRpcMessageWithId.cs

@@ -26,5 +26,5 @@ public abstract class JsonRpcMessageWithId : JsonRpcMessage
     /// Each ID is expected to be unique within the context of a given session.
     /// </remarks>
     [JsonPropertyName("id")]
-    public required RequestId Id { get; set; }
+    public RequestId Id { get; set; }
 }