chore: import upstream snapshot with attribution

dotnet/samples/Demos/StepwisePlannerMigration/Models/PlanRequest.cs

@@ -0,0 +1,14 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+namespace StepwisePlannerMigration.Models;
+
+/// <summary>
+/// Request model for planning endpoints.
+/// </summary>
+public class PlanRequest
+{
+    /// <summary>
+    /// A goal which should be achieved after plan execution.
+    /// </summary>
+    public string Goal { get; set; }
+}