refactor(mesh): step 4 — drop the receptionist extension and akka.cluster.client from akka.conf

akka.extensions loses ClusterClientReceptionistExtensionProvider and the whole
akka.cluster.client block (buffer-size / reconnect-timeout / receptionist.role) goes with
it. The DistributedPubSub extension and the maximum-frame-size / log-frame-size-exceeding
canary both stay — DPS is the pair-local transport and the frame canary guards every
remoting message, neither of which the cross-mesh deletion touches.

MeshTransportHoconTests drops its three ClusterClientSettings/ClusterReceptionistSettings
asserts and keeps the frame-size pin and the DPS-extension pin, renamed since it no longer
has a receptionist to survive alongside.

tests/Core/ZB.MOM.WW.OtOpcUa.Cluster.Tests/MeshTransportHoconTests.cs

@@ -1,5 +1,4 @@
 using Akka.Actor;
-using Akka.Cluster.Tools.Client;
 using Akka.Configuration;
 using Shouldly;
 using Xunit;
@@ -7,8 +6,8 @@ using Xunit;
 namespace ZB.MOM.WW.OtOpcUa.Cluster.Tests;
 
 /// <summary>
-///     Asserts the ClusterClient / receptionist / frame-size settings that are actually <b>in
-///     force</b> on a real <see cref="ActorSystem"/> built from the shipped base config.
+///     Asserts the remoting frame-size canary and the DistributedPubSub extension that are actually
+///     <b>in force</b> on a real <see cref="ActorSystem"/> built from the shipped base config.
 /// </summary>
 /// <remarks>
 ///     Asserting the <c>akka.conf</c> text instead would pass while a competing HOCON fragment
@@ -29,38 +28,6 @@ public class MeshTransportHoconTests : IDisposable
         _sys = ActorSystem.Create("mesh-hocon-probe", config);
     }
 
-    [Fact]
-    public void Cluster_client_buffering_is_disabled()
-    {
-        // Asserted through ClusterClientSettings — the object the client is actually built from —
-        // NOT through Config.GetInt. GetInt returns 0 for a MISSING key, so a config assertion here
-        // passes identically whether the setting is explicitly 0 or absent with Akka's default of
-        // 1000 in force. It was green before this feature existed; that is the whole defect class.
-        //
-        // buffer-size = 0 means "drop now if the receptionist is unknown". Buffering would replay a
-        // DispatchDeployment on reconnect and apply a deployment the coordinator already sealed as
-        // TimedOut, leaving the node running a configuration the database says failed. This is a
-        // behaviour decision, not a tuning knob.
-        ClusterClientSettings.Create(_sys).BufferSize.ShouldBe(0);
-    }
-
-    [Fact]
-    public void Cluster_client_retries_forever_rather_than_self_stopping()
-    {
-        // reconnect-timeout = off surfaces as a null TimeSpan on the settings object. A central node
-        // down for an hour must not require a driver-node restart to become reachable again.
-        ClusterClientSettings.Create(_sys).ReconnectTimeout.ShouldBeNull();
-    }
-
-    [Fact]
-    public void Receptionist_serves_every_member_regardless_of_role()
-    {
-        // An empty role is what makes "registered per node, not as a singleton" work: every member
-        // hosts a receptionist, so ClusterClient contact rotation reaches whichever node answers.
-        // Scoping this to a role would make the boundary reachable through one node only.
-        ClusterReceptionistSettings.Create(_sys).Role.ShouldBeNull();
-    }
-
     [Fact]
     public void Oversized_frames_are_logged_rather_than_dropped_silently()
     {
@@ -73,21 +40,11 @@ public class MeshTransportHoconTests : IDisposable
     }
 
     [Fact]
-    public void Receptionist_extension_is_registered_at_startup()
+    public void Distributed_pub_sub_extension_is_registered_at_startup()
     {
-        // Not auto-started. Without this entry the receptionist materialises only if some code path
-        // happens to call ClusterClientReceptionist.Get(system) first, which makes "is the boundary
-        // listening?" depend on startup ordering rather than on configuration.
-        _sys.Settings.Config.GetStringList("akka.extensions")
-            .ShouldContain(s => s.Contains("ClusterClientReceptionistExtensionProvider"));
-    }
-
-    [Fact]
-    public void Distributed_pub_sub_extension_survives_alongside_the_receptionist()
-    {
-        // The dark switch keeps both transports live for one phase, and DPS additionally carries
-        // traffic Phase 2 does not touch at all — node-local alarm-command publishes and the
-        // node<->node redundancy-state probe leg. Losing it here would break far more than deploys.
+        // DPS is PAIR-LOCAL and survives the cross-mesh transport's deletion: it carries
+        // MeshTransport:Mode = Dps (the single-mesh default), the node-local alarm-command publishes
+        // and the node<->node redundancy-state probe leg. Losing it would break far more than deploys.
         _sys.Settings.Config.GetStringList("akka.extensions")
             .ShouldContain(s => s.Contains("DistributedPubSubExtensionProvider"));
     }