fix: add UseTestServer() to HostBuilder configuration

The migration from WebHostBuilder to HostBuilder omitted the
.UseTestServer() call, causing the test host to create a real
Kestrel server instead of a TestServer. This resulted in an
InvalidCastException when GetTestServer() tried to cast
KestrelServerImpl to TestServer.

Added .UseTestServer() in both the StartupClass and Middleware
configuration paths.

Signed-off-by: Shashwat Sinha <shashwat.sinha2003@gmail.com>

Author: ShashwatSinha03

src/Lucene.Net.Tests.Replicator/ReplicatorTestCase.cs

@@ -89,6 +89,7 @@ public static TestServer NewHttpServer(IReplicationService service, MockErrorCon
                 var hostBuilder = new HostBuilder()
                     .ConfigureWebHostDefaults(webBuilder =>
                     {
+                        webBuilder.UseTestServer();
                         webBuilder.ConfigureServices(container =>
                         {
                             container.AddSingleton(service);
@@ -115,6 +116,7 @@ public static TestServer NewHttpServer(IReplicationService service, MockErrorCon
                 var hostBuilder = new HostBuilder()
                     .ConfigureWebHostDefaults(webBuilder =>
                     {
+                        webBuilder.UseTestServer();
                         webBuilder.ConfigureServices(container =>
                         {
                             container.AddRouting();