Modernize shared client controls: Controls.Net4 (Managed Session + new Subscription Engine)

[romanett]

Sub-issue of #755.

Why

Samples/Controls.Net4 ("UA Sample Controls") supplies the session tree, browse tree, subscription and monitored-item dialogs used by the UA Sample Client (Samples/Client.Net4). It still uses raw Session and the classic Subscription/MonitoredItem engine (see SubscriptionDlg.cs: new Subscription(session.DefaultSubscription), session.AddSubscription, per-item Notification).

Scope (files)

• Sessions/SessionTreeCtrl.cs
• Sessions/BrowseTreeCtrl.cs
• Subscriptions/SubscriptionDlg.cs
• Subscriptions/CreateMonitoredItemsDlg.cs
• Subscriptions/MonitoredItemDlg.cs
• Subscriptions/MonitoredItemConfigCtrl.cs

Tasks

1. Managed Session: establish/own connections through ManagedSession (via ManagedSessionFactory/ISessionFactory) with built-in reconnect; keep browse/read surfaces typed as ISession.
2. New Subscription Engine: migrate subscription creation and monitored-item add/remove to the V2 ISubscriptionManager/IManagedSubscription engine (MonitoredItems.TryAdd(name, MonitoredItemOptions, out _), MonitoredItems.TryRemove(handle), ISubscriptionNotificationHandler). Preserve the tree/dialog UX (state, publish-status display).
3. Keep control public APIs source-compatible where feasible; note breaking changes in the PR.

References

• https://github.com/OPCFoundation/UA-.NETStandard/blob/master/docs/Sessions.md
• https://github.com/OPCFoundation/UA-.NETStandard/blob/master/docs/Subscriptions.md

Acceptance criteria

• Controls.Net4 builds against 2.0.x.
• No classic-engine or SessionReconnectHandler usage remains in these files.
• The UA Sample Client can browse, create a subscription, add/remove monitored items, and see live updates through the modernized controls.

Dependencies

None — foundation issue for the UA Sample Client. Can proceed in parallel with the ClientControls.Net4 foundation issue.