refactor(mesh): step 1 — delete the central ClusterClient factory and per-cluster client cache

IMeshClusterClientFactory/DefaultMeshClusterClientFactory are gone, along with
CentralCommunicationActor's _clients map, RebuildClient, the SendToAll fan and the
receptionist-address half of the contact load. LoadContactsFromDb SURVIVES — it feeds the
gRPC dialer's endpoint resolution and the known-cluster gate — but now projects only the
columns those two need (the AkkaPort/ActorPath.TryParse parse and its Malformed list went
with the receptionist addresses).

Ripples fixed in this step because the build must be green at every commit:
  - ControlPlane/ServiceCollectionExtensions drops the factory argument.
  - CentralCommunicationActorTests: the six ClusterClient-only facts are deleted (each has a
    Grpc counterpart that already asserted the same property independently), the three
    transport-agnostic facts move to Grpc mode, and the Dps fact keeps its own coverage.
  - MeshClusterClientFactoryTests deleted (its subject is gone).
  - MeshClusterClientBoundaryTests deleted here rather than in step 7: it drives
    DefaultMeshClusterClientFactory directly, so it cannot compile past this commit. Its
    replacement, MeshGrpcBoundaryTests, lands in step 7 and keeps the load-bearing assertion
    (two separate meshes, no cross-mesh Akka association).

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

@@ -56,11 +56,11 @@ public static class ServiceCollectionExtensions
 
         // Per-cluster mesh Phase 2: the central end of the central↔node command boundary.
         //
-        // A plain WithActors, NOT a singleton — and that is the load-bearing part. A driver node's
-        // ClusterClient rotates across contact points and must find a live comm actor at whichever
-        // central node answers. As a singleton it would exist on one central node only, and the
-        // rotation would fail silently against the other: acks would land sometimes, depending on
-        // which contact the client happened to settle on.
+        // A plain WithActors, NOT a singleton — and that is the load-bearing part. A driver node
+        // walks MeshTransport:CentralGrpcEndpoints in order and posts its ack to whichever central
+        // node answers first, so a live comm actor must exist on both. As a singleton it would exist
+        // on one central node only, and the other node's ack ingress would have nothing to forward
+        // to: acks would land sometimes, depending on which endpoint answered.
         builder.WithActors((system, registry, resolver) =>
         {
             var dbFactory = resolver.GetService<IDbContextFactory<OtOpcUaConfigDbContext>>();
@@ -85,7 +85,6 @@ public static class ServiceCollectionExtensions
                 CentralCommunicationActor.Props(
                     dbFactory,
                     options,
-                    new DefaultMeshClusterClientFactory(),
                     // Resolved lazily, per message, so this actor never depends on the order in
                     // which Akka.Hosting materialises the singleton proxy relative to this block.
                     () => registry.TryGet<ConfigPublishCoordinatorKey>(out var coordinator)