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.

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Communication/EventStreamCommandDeliveryTests.cs

@@ -50,7 +50,7 @@ public class EventStreamCommandDeliveryTests : RuntimeActorTestBase
         var driverHost = Sys.ActorOf(
             DriverHostActor.Props(NewInMemoryDbFactory(), new NodeId("127.0.0.1:4053"), ackRouter: ackRouter.Ref),
             "driver-host");
-        var comm = Sys.ActorOf(NodeCommunicationActor.Props(centralClient: null), "node-communication");
+        var comm = Sys.ActorOf(NodeCommunicationActor.Props(), "node-communication");
 
         // Warm-up: a direct Tell whose ack proves PreStart has completed, and with it the
         // EventStream subscription. Without this the relay below races the subscription.
@@ -141,7 +141,7 @@ akka {
             ScriptedAlarmHostActor.Props(
                 CreateTestProbe().Ref, CreateTestProbe().Ref, upstream, engine, new NodeId("node-A")),
             "scripted-alarm-host");
-        var comm = Sys.ActorOf(NodeCommunicationActor.Props(centralClient: null), "node-communication");
+        var comm = Sys.ActorOf(NodeCommunicationActor.Props(), "node-communication");
 
         var cmd = new AlarmCommand("alarm-unowned", "Acknowledge", "alice", "c", null);