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/Adapters/DefaultSessionAdapter.cs

@@ -115,22 +115,6 @@ internal sealed class DefaultSessionAdapter : ISessionAdapter
         return (nextCp, nextRefs ?? []);
     }
 
-    /// <inheritdoc />
-    public async Task<bool> HasChildrenAsync(NodeId nodeId, CancellationToken ct)
-    {
-        var (_, _, references) = await _session.BrowseAsync(
-            null,
-            null,
-            nodeId,
-            1u,
-            BrowseDirection.Forward,
-            ReferenceTypeIds.HierarchicalReferences,
-            true,
-            0u);
-
-        return references != null && references.Count > 0;
-    }
-
     /// <inheritdoc />
     public async Task<Models.HistoryUpdateOutcome> HistoryUpdateAsync(HistoryUpdateDetails details, CancellationToken ct)
     {