fix(ReplicatorTestCase): Start host before GetTestServer()

When using HostBuilder, GetTestServer() requires the host to be
explicitly started first — unlike the legacy WebHostBuilder which
started it implicitly. Add host.Start() before GetTestServer()
calls in both branches (startup class and middleware).

Author: ShashwatSinha03

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

@@ -107,6 +107,7 @@ public static TestServer NewHttpServer(IReplicationService service, MockErrorCon
                     });
 
                 var host = hostBuilder.Build();
+                host.Start();
                 var server = host.GetTestServer();
                 server.BaseAddress = new Uri("http://localhost" + ReplicationService.REPLICATION_CONTEXT);
                 return server;
@@ -167,6 +168,7 @@ public static TestServer NewHttpServer(IReplicationService service, MockErrorCon
                     });
 
                 var host = hostBuilder.Build();
+                host.Start();
                 return host.GetTestServer();
             }
         }