Migrate Opc.Ua.Sample library Node Managers (TestData, Boiler, MemoryBuffer) to AsyncCustomNodeManager + Fluent

[romanett]

Part of #753.

Goal

Migrate the shared Opc.Ua.Sample library node managers (used by the WinForms Sample Server and others) to AsyncCustomNodeManager, and adopt source generation / FluentNodeManager where a generated model exists.

Scope

• Samples/Opc.Ua.Sample/TestData/TestDataNodeManager.cs — TestDataNodeManager : CustomNodeManager2 (generated TestData model)
• Samples/Opc.Ua.Sample/Boiler/BoilerNodeManager.cs — BoilerNodeManager : SampleNodeManager (generated Boiler model)
• Samples/Opc.Ua.Sample/MemoryBuffer/MemoryBufferNodeManager.cs — MemoryBufferNodeManager : SampleNodeManager (hand-built)
• Local base classes: Samples/Opc.Ua.Sample/Base/CustomNodeManager.cs (local CustomNodeManager2), Samples/Opc.Ua.Sample/Base/SampleNodeManager.cs
• Samples/Opc.Ua.Sample/Opc.Ua.Sample.csproj (already references the source generator)

Because these managers derive from local base classes bundled in this project, decide up front whether to (a) retire the local CustomNodeManager2/SampleNodeManager in favor of the library AsyncCustomNodeManager, or (b) keep thin local shims. Prefer (a).

Step 1 — AsyncCustomNodeManager (all three managers)

1. Rebase each manager onto the library AsyncCustomNodeManager; constructor base(server, configuration, <namespaceUri>).
2. CreateAddressSpace → override async ValueTask CreateAddressSpaceAsync(...); LoadPredefinedNodes → LoadPredefinedNodesAsync.
3. Convert method/read/write callbacks to async handlers; TestData's system-callback simulation should push updates via await ClearChangeMasksAsync. MemoryBuffer: convert the buffer scan/update path to the async monitored-item/notification path.
4. Remove lock (Lock) blocks — use base async locking.
5. Register via IAsyncNodeManagerFactory; update Samples/Server.Net4 (UA Sample Server) startup accordingly.

Step 2 — FluentNodeManager (generated models: TestData, Boiler)

• Opt in to generated node managers for the TestData and Boiler designs via [NodeManager] / MSBuild flag; use the generated LoadPredefinedNodesAsync.
• Move callbacks into partial void Configure(INodeManagerBuilder builder); delete redundant hand-written address-space code. (MemoryBuffer stays a hand-written AsyncCustomNodeManager.)

Acceptance criteria

• TestDataNodeManager, BoilerNodeManager, MemoryBufferNodeManager all derive from AsyncCustomNodeManager; local sync base classes are removed or reduced to non-conflicting helpers; no lock (Lock) remains.
• Opc.Ua.Sample builds; the UA Sample Server (Samples/Server.Net4) starts; a client browses/reads/writes/subscribes across TestData, Boiler and MemoryBuffer, and TestData method calls succeed.

References

• docs/AsyncServerSupport.md, docs/NodeManagers.md (UA-.NETStandard).