Fix NetCoreTestApp to use multi-tenant app from lab

- Use LabUserHelper.GetDefaultUserWithMultiTenantAppAsync() to get correct multi-tenant app ID
- Resolves AADSTS50194 error when using /common endpoint with single-tenant app
- Ensures proper multi-tenant configuration for testing scenarios
- Added LabInfrastructure using statement for lab integration

Author: RyAuld

tests/devapps/NetCoreTestApp/Program.cs

@@ -16,6 +16,7 @@
 using Microsoft.Identity.Client.Broker;
 using Microsoft.Identity.Client.Extensibility;
 using Microsoft.Identity.Test.Integration.NetFx.Infrastructure;
+using Microsoft.Identity.Test.LabInfrastructure;
 using NetCoreTestApp.Experimental;
 
 namespace NetCoreTestApp
@@ -28,8 +29,8 @@ public class Program
               { "caller-sdk-ver", "123" }
           };
 
-        // This app has http://localhost redirect uri registered
-        private static readonly string s_clientIdForPublicApp = "1d18b3b0-251b-4714-a02a-9956cec86c2d";
+        // This app will be dynamically set to the multi-tenant app from lab
+        private static readonly string s_clientIdForPublicApp = LabUserHelper.GetDefaultUserWithMultiTenantAppAsync().Result.App.AppId;
 
         private static readonly string s_username = ""; // used for WIA and U/P, cannot be empty on .net core