Move the user interface free half of the sample clients into one library

[romanett]

Proposed changes

Works through the follow-ups #865 recorded when the Workshop client split stopped where it did. Doing so turned up more duplication than the issue knew about, so a lot of this is deletion.

Workshop/Common was the surprise. Its ModelUtils was a second copy of the instance declaration walk in ClientUtils — and the HistoricalEvents client model used it, so a model depended on a Windows Forms assembly, which is the one thing the model tier exists to prevent. Both copies are now SampleTypeModel. FormUtils duplicated SampleSession; FilterDefinition and SelectLocaleDlg were stale forks; ParsedNodeId duplicated Opc.Ua.Server.ParsedNodeId, which is the type the sample servers were resolving to anyway, so nothing referenced it. The Quickstart library is now only the backward-compatibility shim it is named for.

The connection state machine left ConnectServerCtrl. It lived inside a UserControl, so the only way to open the kind of session the samples run on was to create one; the headless tests went through SampleSessionFactory instead and exercised a second implementation of the same sequence. SampleConnection now owns discovery, the session, the reconnect and the bounded close, and the control is the tool bar over it. Two things stayed with the window deliberately: asking a person about an untrusted certificate, and thread marshalling — a form closing blocks its own message loop, so an event posted to that loop arrives after the form is gone, while one raised inline still reaches it.

The two subscribe wizards had drifted copies of the same subscription plumbing; their status strings disagreed. SampleSubscription backs both, on the more informative of the two formats. SampleBrowser, SampleHistory and SampleDiscovery take the remaining protocol out of the browse tree, the history view and the endpoint editor.

Each Workshop client gained a composition root, Add<Sample>Client(), the counterpart of the Add<Sample>Server() of its server half, and its window takes the model through its constructor. Tier 2 builds each form through that same registration rather than calling the constructor, so a client which starts taking something else from the container needs no change to the factory table.

The Global Discovery Client kept its logic in three panels and now has a Model/ namespace like the Workshop clients (RegistrationModel, CertificateModel, TrustListModel), covered by ModelContractTests. It does not build on SampleClientModel — it has no session a connect control hands over, it works through the GDS, push-configuration and discovery clients of the stack — so the contract test learned to check a client's models without demanding that base class. The two questions only a person can answer reach the models as delegates: whether an existing certificate file may be overwritten, and which record is meant when the GDS holds several for one application URI.

Windows now dispose their model from Dispose(bool), which needed a synchronous Dispose on the base and removed all 18 CA2213 suppressions the issue counted. That turned the analyzer loose on the models' own fields and found a real leak — the refresh semaphore of StateMachinesClientModel was never disposed. Two more in the GDS certificate path are fixed rather than suppressed.

Related Issues

• Fixes Client models: the follow-ups left open by the Workshop client split #865

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

Test results. Tier 0 237/237, tier 1 121/121, tier 1.5 178/178, tier 1.7 420/420, tier 2 65/65. Build warning count is unchanged from master.

This branch also removes a debug leftover from #864 that was making every server-side tier slow: a ProbeFileLoggerProvider in Tests/Samples.Tests.Common/SampleServerHost.cs, registered at LogLevel.Trace, doing one File.AppendAllText per traced line under a process-wide lock, plus a stray SampleServerHost.cs.bak. The comment directly above the registration still said "the tests log nothing from the server", which is the tell.

It was making PerfTestClientCountsTheUpdatesItMeasures fail on master. A controlled A/B here, toggling only that registration, gave 3/3 passes at 3-7 s with it off against 3/3 failures at 41-50 s with it on, writing 97 MB in three runs. With it removed, the whole of tier 2 now runs in 2m28s instead of ~6m and passes 65/65.

The fix is cherry-picked from 3d6873a8 (branch romanett/ua-netstandard-836-8b642f), which never reached master.

What was left for its own change. BrowseTreeCtrl and SessionTreeCtrl of Controls.Net4 still mix tree bookkeeping with protocol. SubscriptionHandle moved out of that assembly — it was user-interface-free already, only in the wrong place — but merging it with SampleSubscription would touch ten control files that a single test covers, so the two document each other instead. ReferenceClient and Client.Net4 needed nothing: their forms are 183 and 89 lines and already thin, because their logic was always in the shared control libraries.

Reading order for review. Samples/Client.Common/README.md describes what the library now holds and why the connect control is split the way it is; Samples/Hosting/README.md covers the container registration and the threading reason a model is a singleton; docs/TESTING.md covers the factory table change.

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