Modernize GDS Client (Managed Session + new Subscription Engine)

[romanett]

Sub-issue of #755.

Why

The GDS Client (Samples/GDS/Client) connects to the Global Discovery Server and to managed servers. It should use the v2.0 ManagedSession (built-in reconnect/failover) and the new subscription engine where it monitors nodes.

Scope (files)

• Samples/GDS/Client/MainForm.cs and related forms/controls under Samples/GDS/Client
• Note: references Samples/ClientControls.Net4 and Samples/GDS/ClientControls.

Tasks

1. Managed Session: establish the GDS and server connections via ManagedSession/ManagedSessionFactory with built-in reconnect, replacing raw Session + SessionReconnectHandler usage.
2. New Subscription Engine: migrate any Subscription/MonitoredItem usage (see matches in MainForm.cs) to ISubscriptionManager/IManagedSubscription.
3. Keep GDS push/pull certificate flows working (recent fixes in Add private key password support to GDS Client certificate flows #745/Fix GDS Client Pull injecting GDS server host into certificate domains #749/GDS Client: create a new .pfx (fresh key pair) from a .der certificate #751 must not regress).

References

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

Acceptance criteria

• GDS Client builds and runs against 2.0.x.
• Registration, certificate push/pull, and trust-list flows still work.
• Any monitoring uses the V2 subscription engine; connections reconnect via ManagedSession.

Dependencies

Depends on the ClientControls.Net4 modernization foundation issue.