refactor(mesh): step 2 — delete the node-side ClusterClient and its centralClient parameter

WithOtOpcUaRuntimeActors no longer builds a ClusterClient from
MeshTransport:CentralContactPoints, and NodeCommunicationActor.Props takes only the gRPC
IApplyAckSender. The ackRouter gate narrows from `useClusterClient || useGrpc` to `useGrpc`,
and the CentralClusterClientName constant is gone.

NodeCommunicationActor's no-transport warning now names MeshTransport:CentralGrpcEndpoints
and MeshTransport:Mode only — the old text pointed an operator at a key that no longer
exists.

Ripples in this step: the two ClusterClient-shaped facts in NodeCommunicationActorTests
(Send-not-SendToAll, and sender-wins-over-a-client) are deleted — the first has no subject
left, the second guarded a both-transports-present state that is now unconstructible. The
Runtime ServiceCollectionExtensions fact keeps its ackRouter pin and drops its
"no ClusterClient actor exists" half for the same reason.

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/ServiceCollectionExtensions.cs

@@ -47,7 +47,6 @@ public static class ServiceCollectionExtensions
     public const string OpcUaPublishActorName = "opcua-publish";
     public const string PeerProbeSupervisorName = "peer-probe-supervisor";
     public const string ContinuousHistorizationRecorderActorName = "continuous-historization-recorder";
-    public const string CentralClusterClientName = "central-cluster-client";
 
     /// <summary>
     /// Registers shared runtime services. Currently binds <see cref="IAlarmHistorianSink"/>
@@ -101,8 +100,8 @@ public static class ServiceCollectionExtensions
         // with no gate at all, and both nodes would write every sample twice with nothing to say so.
         services.TryAddSingleton<IHistorizationRoleView>(sp => sp.GetRequiredService<RedundancyRoleView>());
 
-        // ClusterClient→gRPC migration Task 1B.2: the node→central ack transport under
-        // MeshTransport:Mode = Grpc. Registered unconditionally but resolved only by the Grpc arm of
+        // The node→central ack transport under MeshTransport:Mode = Grpc (mesh gRPC command plane,
+        // Task 1B.2). Registered unconditionally but resolved only by the Grpc arm of
         // WithOtOpcUaRuntimeActors — the mode is not knowable here (this runs before configuration is
         // read into options), and a singleton nobody resolves is never constructed, so a Dps node
         // opens no channel.
@@ -404,35 +403,15 @@ public static class ServiceCollectionExtensions
             registry.Register<DependencyMuxActorKey>(mux);
 
             // Per-cluster mesh Phase 2: the node end of the central↔node command boundary. Spawned
-            // BEFORE DriverHostActor because it is the host's ackRouter under ClusterClient mode.
-            // It has no dependency of its own on the host — inbound commands travel via the node's
+            // BEFORE DriverHostActor because it is the host's ackRouter under Grpc mode. It has no
+            // dependency of its own on the host — inbound commands travel via the node's
             // EventStream — so this ordering costs nothing.
             var meshOptions = resolver.GetService<IOptions<MeshTransportOptions>>().Value;
-            var useClusterClient = string.Equals(
-                meshOptions.Mode, MeshTransportOptions.ModeClusterClient, StringComparison.OrdinalIgnoreCase);
             var useGrpc = string.Equals(
                 meshOptions.Mode, MeshTransportOptions.ModeGrpc, StringComparison.OrdinalIgnoreCase);
 
-            IActorRef? centralClient = null;
-            if (useClusterClient)
-            {
-                // Contacts come from appsettings, not the database — the deliberate asymmetry:
-                // central's node set changes as operators add and retire nodes, but central's own
-                // address is part of the deployment. MeshTransportOptionsValidator has already
-                // rejected an empty or malformed list at boot, so this cannot silently produce a
-                // client with no contacts.
-                var contacts = meshOptions.CentralContactPoints
-                    .Select(cp => ActorPath.Parse($"{cp}/system/receptionist"))
-                    .ToImmutableHashSet();
-                centralClient = system.ActorOf(
-                    ClusterClient.Props(ClusterClientSettings.Create(system).WithInitialContacts(contacts)),
-                    CentralClusterClientName);
-            }
-
-            // ClusterClient→gRPC migration Task 1B.2. Deliberately EXCLUSIVE with the branch above —
-            // under Grpc no ClusterClient is created at all, which is half the point of the migration
-            // (it is the cross-WAN Akka association this program removes). The two are never both
-            // handed to the actor, and it prefers the sender if they ever were.
+            // The node→central ack transport. Built ONLY under Grpc: under Dps the ack goes out on
+            // the deployment-acks topic from DriverHostActor and never reaches this actor at all.
             IApplyAckSender? ackSender = null;
             if (useGrpc)
             {
@@ -451,7 +430,7 @@ public static class ServiceCollectionExtensions
             }
 
             var nodeComm = system.ActorOf(
-                NodeCommunicationActor.Props(centralClient, ackSender),
+                NodeCommunicationActor.Props(ackSender),
                 MeshPaths.NodeCommunicationName);
 
             // Registered with the receptionist in ALL modes: under Dps and Grpc nothing dials it, and
@@ -567,12 +546,11 @@ public static class ServiceCollectionExtensions
                     : null;
 
             var driverHost = system.ActorOf(
-                // ackRouter: under ClusterClient AND Grpc the node comm actor relays ApplyAcks across
-                // the boundary (which transport it then uses is its own business — the ClusterClient
-                // ref or the gRPC sender it was built with); under Dps it stays null and the host
-                // publishes on the deployment-acks topic exactly as before. Grpc MUST be included
-                // here: routed to DPS instead, a Grpc node's ack would never reach the sender at all
-                // and every deployment to it would time out while it applied perfectly.
+                // ackRouter: under Grpc the node comm actor relays ApplyAcks across the mesh
+                // boundary through the gRPC sender it was built with; under Dps it stays null and
+                // the host publishes on the deployment-acks topic exactly as before. Routed to DPS
+                // instead, a Grpc node's ack would never reach the sender at all and every
+                // deployment to it would time out while it applied perfectly.
                 // dbFactory is nullable (Phase 4): a driver-only node passes null here — ConfigDb is
                 // gated on hasAdmin and driver-only nodes are forced to FetchAndCache. DriverHostActor
                 // guards every ConfigDb-touching path on it (UpsertNodeDeploymentState no-ops; central
@@ -580,7 +558,7 @@ public static class ServiceCollectionExtensions
                 // alarmStateStore (there is no ConfigDb-backed EF fallback; the host skips the alarm
                 // host when no store was wired).
                 DriverHostActor.Props(dbFactory, roleInfo.LocalNode,
-                    ackRouter: useClusterClient || useGrpc ? nodeComm : null,
+                    ackRouter: useGrpc ? nodeComm : null,
                     driverFactory: driverFactory, localRoles: roleInfo.LocalRoles,
                     dependencyMux: mux,
                     opcUaPublishActor: publishActor,