test(integration): F22 — failover scenario tests + harness Stop/Restart primitives

Extends TwoNodeClusterHarness with three lifecycle primitives:
- StopNodeBAsync()      — graceful CoordinatedShutdown (Cluster.Leave)
- RestartNodeBAsync()   — rebuild node B on same Akka port + same in-memory DB
- WaitForClusterSizeAsync(n) — converge assertion helper

Adds three failover scenario tests:
- Stopping node B shrinks cluster to 1 Up member
- Restarted node B rejoins on the same Akka port
- Deployment started with B down seals with a single NodeDeploymentState
  (validates ConfigPublishCoordinator.DiscoverDriverNodes snapshots
   membership at dispatch time)

Closes follow-up F22. Integration test count: 6 → 9 (+3).

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/TwoNodeClusterHarness.cs

@@ -88,6 +88,58 @@ public sealed class TwoNodeClusterHarness : IAsyncDisposable
         return harness;
     }
 
+    /// <summary>
+    /// Gracefully shuts down node B via <see cref="WebApplication.DisposeAsync"/>, which runs
+    /// CoordinatedShutdown → Cluster.Leave. Node A sees the member transition to Removed within
+    /// a couple of seconds. Use this for failover scenarios; call <see cref="RestartNodeBAsync"/>
+    /// to bring it back on the same Akka port.
+    /// </summary>
+    public async Task StopNodeBAsync()
+    {
+        if (NodeB is null) return;
+        await NodeB.DisposeAsync();
+        NodeB = null!;
+    }
+
+    /// <summary>
+    /// Rebuilds node B on the same Akka port + same in-memory ConfigDb and waits for the cluster
+    /// to re-converge to 2 Up members. Use after <see cref="StopNodeBAsync"/> to test rejoin.
+    /// </summary>
+    public async Task RestartNodeBAsync(TimeSpan? formationTimeout = null)
+    {
+        NodeB = await BuildNodeAsync(
+            host: LoopbackHost,
+            akkaPort: NodeBAkkaPort,
+            seedHost: LoopbackHost,
+            seedAkkaPort: NodeAAkkaPort,
+            dbName: SharedDbName);
+
+        await WaitForClusterFormationAsync(
+            NodeASystem,
+            NodeBSystem,
+            formationTimeout ?? TimeSpan.FromSeconds(20));
+    }
+
+    /// <summary>
+    /// Waits for node A's cluster view to reach <paramref name="expectedUpMembers"/> members in
+    /// <see cref="MemberStatus.Up"/>. Used for asserting shrink-after-stop or grow-after-restart.
+    /// </summary>
+    public async Task WaitForClusterSizeAsync(int expectedUpMembers, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (DateTime.UtcNow < deadline)
+        {
+            var count = Akka.Cluster.Cluster.Get(NodeASystem).State.Members
+                .Count(m => m.Status == MemberStatus.Up);
+            if (count == expectedUpMembers) return;
+            await Task.Delay(200);
+        }
+        var actual = Akka.Cluster.Cluster.Get(NodeASystem).State.Members
+            .Count(m => m.Status == MemberStatus.Up);
+        throw new TimeoutException(
+            $"Cluster did not converge to {expectedUpMembers} Up members within {timeout}. Actual={actual}");
+    }
+
     private static async Task<WebApplication> BuildNodeAsync(
         string host, int akkaPort, string seedHost, int seedAkkaPort, string dbName)
     {