chore: adds missing using

Signed-off-by: Vincent Biret <vibiret@microsoft.com>

Author: baywet

test/Microsoft.OpenApi.Tests/Models/OpenApiServerTests.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license.
 
 using System.Collections.Generic;
+using System.Text.Json.Nodes;
 using System.Threading.Tasks;
 using Xunit;