docs(mesh): correct the all-malformed-cluster comments in CentralCommunicationActor

The per-cluster contact map only gains a key when a row parses, so an
all-malformed cluster has no key at all and is caught by the absent-key arm;
the empty-set guard is defensive/unreachable for the DB producer. Comment-only
(Task 4 code-review minor follow-up).

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Server/ZB.MOM.WW.OtOpcUa.ControlPlane/Communication/CentralCommunicationActor.cs

@@ -258,8 +258,9 @@ public sealed class CentralCommunicationActor : ReceiveActor, IWithTimers
             kv => kv.Key, kv => kv.Value.ToImmutableHashSet());
 
         // Clusters that dropped out of the DB entirely, or whose only rows were malformed this refresh:
-        // stop and forget their clients so a vanished cluster stops receiving. A cluster whose rows all
-        // went malformed lands here via the empty-set branch.
+        // stop and forget their clients so a vanished cluster stops receiving. LoadContactsFromDb only
+        // creates a ByCluster key when at least one of its rows parsed, so an all-malformed cluster has
+        // NO key at all and is caught here by the absent-key (!TryGetValue) arm — not by an empty value.
         var gone = _clients.Keys
             .Where(id => !byCluster.TryGetValue(id, out var c) || c.IsEmpty)
             .ToList();
@@ -277,8 +278,10 @@ public sealed class CentralCommunicationActor : ReceiveActor, IWithTimers
         {
             if (contacts.IsEmpty)
             {
-                // Only-malformed rows for this cluster this refresh. Warn per cluster; its client (if
-                // any) was already stopped above.
+                // Defensive: LoadContactsFromDb never emits an empty-but-present cluster (a key is
+                // created only on a first successfully-parsed row), so this arm is unreachable for the
+                // DB producer — an all-malformed cluster is handled by the absent-key branch above. It
+                // guards a ContactsLoaded (a public record) constructed elsewhere with an empty list.
                 _log.Warning(
                     "Cluster {ClusterId} has no usable ClusterNode contact points; no ClusterClient "
                     + "was created for it and its nodes' commands will be dropped until a refresh finds "