perf(client): lazy child detection in BrowseAsync, NodeId-indexed UI notifications

R08 3.6: BrowseAsync no longer issues one HasChildren Browse per Object child; an
Object reports HasChildren (may have children) and is confirmed when expanded or
recursed. ISessionAdapter.HasChildrenAsync is removed. The UI subscriptions panel
routes each notification through a NodeId index kept in step with
ActiveSubscriptions (add/remove/replace/reset) instead of scanning every row.

Refs #747
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

src/Client/ZB.MOM.WW.OtOpcUa.Client.Shared/OpcUaClientService.cs

@@ -224,18 +224,15 @@ public sealed class OpcUaClientService : IOpcUaClientService
 
         while (references.Count > 0)
         {
+            // Child detection is lazy: an Object MAY have children and is confirmed when the caller expands or
+            // recurses into it. Probing each Object child with its own Browse made one level of N folders cost N + 1
+            // round trips, and a recursive subscribe over a 50k-node tree ~50k extra Browse calls.
             foreach (var reference in references)
-            {
-                var childNodeId = ExpandedNodeId.ToNodeId(reference.NodeId, _session.NamespaceUris);
-                var hasChildren = reference.NodeClass == NodeClass.Object &&
-                                  await _session.HasChildrenAsync(childNodeId, ct);
-
                 results.Add(new BrowseResult(
                     reference.NodeId.ToString(),
                     reference.DisplayName?.Text ?? string.Empty,
                     reference.NodeClass.ToString(),
-                    hasChildren));
-            }
+                    reference.NodeClass == NodeClass.Object));
 
             if (continuationPoint is not { Length: > 0 } pending) break;
             (continuationPoint, references) = await _session.BrowseNextAsync(pending, ct);