test(communication): close register row 102 — serialize the two gauge-moving classes so the subscribers.active baseline holds

The gauge test polls baseline+1, but the process-global gauge's baseline is
captured once: a parallel class's streams closing after capture lowers the
gauge below baseline+1 forever. Survey confirmed only SiteStreamGrpcServerTests
and CleanupVerificationTests drive the real SubscribeInstance handler that
moves the counter; both now share the SiteStreamSubscriberGauge collection
(row-96 pattern). 4/4 full-suite runs green, durations unchanged.

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/Grpc/CleanupVerificationTests.cs

@@ -11,6 +11,10 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Tests.Grpc;
 /// Verifies that after a gRPC stream is cancelled, the SiteStreamManager
 /// subscription is properly cleaned up with no leaked subscriptions.
 /// </summary>
+// Serialized with SiteStreamGrpcServerTests (register row 102) — this class's real
+// SiteStreamGrpcServer streams move the process-global subscribers.active gauge that
+// SiteStreamSubscribersActiveGauge_GoesToOneOnConnect_AndBackToZeroOnCancel baselines.
+[Collection("SiteStreamSubscriberGauge")]
 public class CleanupVerificationTests : TestKit
 {
     [Fact]