Drop the per-sample StandardServer subclasses for a builder-only DI surface

[romanett]

Proposed changes

The DI surface of the server samples is now one shape, and no sample has a StandardServer subclass any more.

Every Add<Sample>Server composition root is

services.AddSampleServer(
    configurationFile ?? ConfigurationFile,
    server => server.AddNodeManager<BoilerNodeManagerFactory>(),
    configure);

The server behind it is one shared SampleServer in Samples/Hosting - the stack's DependencyInjectionStandardServer, whose only override derives ServerProperties from the configuration. The 13 samples whose server class did nothing but name the product (Boiler, AlarmCondition, DataAccess, DataTypes, Empty, FileTransfer, HistoricalAccess, Methods, NodeManagement, PerfTest, SimpleEvents, StateMachines, Views) lost it outright.

What the other server classes overrode became a registration on the server builder of the stack:

Was	Is
CreateRoleManager + AddIdentity (RoleManagement, AliasNames)	server.ConfigureRoles(...) of the stack
OnServerStarted registering authenticators (RoleManagement, AliasNames, UserAuthentication, UA sample)	server.AddIdentityAuthenticator((sp, validator) => new UserNamePasswordAuthenticator(...))
OnNodeManagerStartedAsync setting the history capabilities (HistoricalEvents)	server.AddStartupTask<HistoricalEventsCapabilities>(), an IServerStartupTask
CreateResourceManager translations (UserAuthentication)	server.AddStartupTask<UserAuthenticationTranslations>()
server factory adding one node manager per configured endpoint (Aggregation)	server.AddNodeManagers(provider => ...), run with the configuration loaded
LoadServerProperties everywhere	derived from ApplicationName / ProductUri of the configuration

The samples built around their ApplicationInstance (UA sample server, UA sample client, GDS WinForms server) run their server on the same stack pipeline: SampleApplication implements the stack's IOpcUaApplicationConfigurationProvider, and AddSampleServer(server => ...) without a file hosts the server on it. That removes the IServerBase path, AddSampleNodeManager<T>(), the Func<IServiceProvider, TServer> overloads and two unused options from the hosting library; SampleApplicationHostedService only loads the configuration eagerly for the forms now.

Three AddSampleServer<TServer> users remain, each for a reason stated at the call site: AliasNames (AliasNamesServer : SampleServer registers and materializes the standard TagVariables alias store before the address space is built, which the stack's AddAliasNameStore does too late), ReferenceServer (the quickstart library's own class), GDS (the server takes databases and certificate groups and is registered with AddSingleton(CreateServer) first).

The helpers the stack lacks are one-liners over its public registration types in SampleServerBuilderExtensions (AddNodeManager(instance), AddNodeManagers(factories), AddIdentityAuthenticator(factory), AddStartupTask<T>()). The corresponding SDK gaps are filed as OPCFoundation/UA-.NETStandard#4410; once they land, SampleServer, those helpers and the AliasNamesServer subclass can go.

Behavior changes a reviewer should know about:

• Server-side reverse connect is gone for the Aggregation and UA sample servers: DependencyInjectionStandardServer is not a ReverseConnectServer. Both configurations had no reverse connect clients enabled; the console aggregation server no longer prints that section.
• Product names now come from the configuration (ApplicationName, ProductUri) instead of per-sample strings.
• The UA sample's X509 check is the stack's X509Authenticator against the Users trust list. The UserAuthentication sample lost its dead impersonation stubs (#if TODO, Kerberos placeholders).
• The reference server entry point lost an unused server factory left over from the previous migration.

Also documented in Samples/Hosting/README.md.

Related Issues

• Follows up Register the sample node managers through DI and finish the node manager migrations preview.4 unlocks #856 (node managers through DI)
• SDK gaps: Hosted server: remaining StandardServer hooks without a DI seam (ServerProperties, authenticator factories, startup tasks, node manager instances, alias materialization, reverse connect) UA-.NETStandard#4410

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

Verified locally: full UA Samples.slnx build with no new warnings; SampleServers.Tests 115 passed (hosts every sample server, the GDS and aggregation consoles included); SampleNodeManagers.Tests 160 passed, 3 skipped as before (roles, alias names, user authentication, event history capabilities); HostedServerBootstrapTests 3 passed, including the new AServerRunsOnTheApplicationInstanceOfTheSample, which covers the application-instance path the UA sample and GDS servers now use.

Alternatives considered: a shared IMainNodeManagerFactory decorator seam in the hosting library for AliasNames (rejected, one sample does not justify an invented seam), and keeping ReverseConnectServer as the shared base (rejected, it would lose the DI hooks of DependencyInjectionStandardServer for everyone).

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