Modernize core Samples clients: ReferenceClient + UA Sample Client (Managed Session + new Subscription Engine)

[romanett]

Sub-issue of #755.

Why

The two headline desktop clients — ReferenceClient (Samples/ReferenceClient) and UA Sample Client (Samples/Client.Net4) — should demonstrate the v2.0 client stack: a ManagedSession with built-in reconnect and the new subscription engine, consuming the already-modernized shared controls.

Scope (files)

• Samples/ReferenceClient/MainForm.cs, Program.cs (uses ClientControls.Net4)
• Samples/Client.Net4/SampleClientForm.cs, Program.cs (uses Controls.Net4 + ClientControls.Net4)

Tasks

1. Managed Session: connect through ManagedSession/ManagedSessionFactory; remove any app-level SessionReconnectHandler bookkeeping now handled by the managed session. Keep the ISession m_session field type on read/browse paths.
2. New Subscription Engine: any subscription/monitored-item code owned directly by these forms (not delegated to shared controls) moves to ISubscriptionManager/IManagedSubscription.
3. Verify the samples against the Quickstart Reference Server (Samples/ReferenceServer): connect, browse, subscribe, reconnect after a server restart.

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

• Both apps build and run against 2.0.x.
• Connection uses ManagedSession; reconnect works after dropping the server.
• Subscriptions deliver notifications through the V2 engine.

Dependencies

Depends on the shared-control foundation issues:

• ClientControls.Net4 modernization (for ReferenceClient and Client.Net4)
• Controls.Net4 modernization (for Client.Net4)