fix(redundancy): periodic heartbeat re-publish so late subscribers learn their role

tests/Server/ZB.MOM.WW.OtOpcUa.ControlPlane.Tests/RedundancyStateActorTests.cs

@@ -43,6 +43,29 @@ public sealed class RedundancyStateActorTests : ControlPlaneActorTestBase
         probe.ExpectNoMsg(TimeSpan.FromMilliseconds(500));
     }
 
+    /// <summary>
+    /// Verifies the periodic heartbeat re-publishes the CURRENT snapshot even when no new cluster
+    /// events arrive — so a late subscriber that missed the change-driven publish converges within
+    /// the heartbeat interval. The first message is the self-join publish; the second proves the
+    /// recurring heartbeat fires with no intervening cluster event.
+    /// </summary>
+    [Fact]
+    public void Heartbeat_republishes_current_snapshot_without_cluster_events()
+    {
+        var probe = CreateTestProbe("heartbeat-listener");
+        Sys.ActorOf(
+            RedundancyStateActor.Props(
+                broadcast: msg => probe.Ref.Tell(msg),
+                heartbeatInterval: TimeSpan.FromMilliseconds(300)),
+            "redundancy-heartbeat");
+
+        // First publish: the change-driven self-join snapshot.
+        probe.ExpectMsg<RedundancyStateChanged>(TimeSpan.FromSeconds(2));
+
+        // Second publish: the periodic heartbeat re-publish, with NO new cluster event sent.
+        probe.ExpectMsg<RedundancyStateChanged>(TimeSpan.FromSeconds(2));
+    }
+
     /// <summary>
     /// Regression guard: the snapshot node id MUST be the canonical <c>host:port</c> form (matching
     /// ClusterRoleInfo.LocalNode/ToNodeId), or every consumer's