fix: route debug stream events through ClusterClient site→central path

ClusterClient Sender refs are temporary proxies — valid for immediate reply
but not durable for future Tells. Events now flow as DebugStreamEvent through
SiteCommunicationActor → ClusterClient → CentralCommunicationActor → bridge
actor (same pattern as health reports). Also fix DebugStreamHub to use
IHubContext for long-lived callbacks instead of transient hub instance.

tests/ScadaLink.SiteRuntime.Tests/Actors/InstanceActorIntegrationTests.cs

@@ -40,6 +40,22 @@ public class InstanceActorIntegrationTests : TestKit, IDisposable
             MaxScriptCallDepth = 10,
             ScriptExecutionTimeoutSeconds = 30
         };
+
+        // Create a fake site-communication actor that unwraps DebugStreamEvent
+        // and forwards the inner event to TestActor (simulating the ClusterClient relay)
+        Sys.ActorOf(Props.Create(() => new DebugStreamEventForwarder(TestActor)), "site-communication");
+    }
+
+    /// <summary>
+    /// Test helper: stands in for SiteCommunicationActor, unwraps DebugStreamEvent
+    /// and forwards the inner event to a target actor.
+    /// </summary>
+    private class DebugStreamEventForwarder : ReceiveActor
+    {
+        public DebugStreamEventForwarder(IActorRef target)
+        {
+            Receive<DebugStreamEvent>(msg => target.Tell(msg.Event));
+        }
     }
 
     void IDisposable.Dispose()