feat(dcl): ServiceLevel-aware OPC UA failover for primary/backup pairs (#138)

Prerequisite for OtOpcUa Warm redundancy (lmxopcua#717): a Warm standby keeps a
healthy session but serves BadOutOfService at a standby ServiceLevel (90/50, boot 1),
so a connection that lands there sat on Bad quality forever.

- OPC UA connections with a backup poll Server_ServiceLevel (i=2267) on the connected
  endpoint (IServiceLevelReadable, implemented by OpcUaDataConnection; never raises
  Disconnected). A level < 200 sustained for the degraded window (default 10 s), or
  the first sample after any (re)connect, triggers a probe of the other endpoint
  (short-lived session: connect, read, dispose). Switch only when it reports a
  Healthy (>= 200) higher level, and not within the minimum dwell (default 60 s) of
  the previous ServiceLevel switch. Both degraded, other unreachable or level
  unknown: stay put. 250/240 (today's OtOpcUa) never starts a streak.
- The switch reuses the failover path: SwitchToOtherEndpoint() now shared by all
  three triggers, then BecomeReconnecting(ReconnectCause.ServiceLevelFailover) -
  zero delay, no unstable accounting, ReSubscribeAll on the new endpoint.
- Config: OpcUaEndpointConfig.ServiceLevelFailover {enabled, degradedWindowSeconds,
  minDwellSeconds}, read from the PRIMARY endpoint's JSON only; null = defaults
  (enabled, 10, 60). Inside the encrypted protocol JSON, so no schema change or
  migration. Central UI: fields in the Backup card next to Failover Retry Count.
  Global: ScadaBridge:DataConnection:ServiceLevelPollInterval (2 s) and
  ServiceLevelProbeTimeout (15 s).
- Tests: actor tests per trigger/non-trigger with positive controls, an unsolicited
  promotion RefreshStart/RefreshEnd test at client and actor level, adapter tests,
  and a real in-process OPC UA server pair end to end. All 15 mutants killed.
- Docs: Component-DataConnectionLayer, components/DataConnectionLayer, Component-CLI.

Claude-Session: https://claude.ai/code/session_01GT67ACmpMR7b9LGRPLijMi

tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/SecureOpcUaTestServer.cs

@@ -18,11 +18,14 @@ namespace ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests.Adapters;
 internal sealed class SecureOpcUaTestServer : IAsyncDisposable
 {
     private readonly ApplicationInstance _app;
+    private readonly StandardServer _server;
     private readonly string _pkiRoot;
 
-    private SecureOpcUaTestServer(ApplicationInstance app, string pkiRoot, string url, X509Certificate2 certificate)
+    private SecureOpcUaTestServer(
+        ApplicationInstance app, StandardServer server, string pkiRoot, string url, X509Certificate2 certificate)
     {
         _app = app;
+        _server = server;
         _pkiRoot = pkiRoot;
         Url = url;
         Certificate = certificate;
@@ -110,16 +113,25 @@ internal sealed class SecureOpcUaTestServer : IAsyncDisposable
             ApplicationType = ApplicationType.Server,
         };
         await app.CheckApplicationInstanceCertificatesAsync(silent: true, lifeTimeInMonths: 12);
-        await app.StartAsync(new TestServer(telemetry, apiKey));
+        var server = new TestServer(telemetry, apiKey);
+        await app.StartAsync(server);
 
         // SDK 2: the loaded application certificate lives in the certificate manager, not on the identifier.
         using var entry = config.CertificateManager.AcquireApplicationCertificateByType(
             ObjectTypeIds.RsaSha256ApplicationCertificateType)
             ?? throw new InvalidOperationException("The test server has no application certificate.");
         var cert = X509CertificateLoader.LoadCertificate(entry.Certificate.RawData);
-        return new SecureOpcUaTestServer(app, pkiRoot, url, cert);
+        return new SecureOpcUaTestServer(app, server, pkiRoot, url, cert);
     }
 
+    /// <summary>
+    /// Sets the server's <c>Server_ServiceLevel</c> (<c>i=2267</c>) — what a redundant server
+    /// publishes for its role (ScadaBridge#138: 250/240 Active, 90/50 Warm standby, 1 booting).
+    /// </summary>
+    /// <param name="level">The ServiceLevel to publish.</param>
+    public void SetServiceLevel(byte level) =>
+        _server.CurrentInstance.ServerObject.ServiceLevel!.Value = level;
+
     /// <inheritdoc />
     public async ValueTask DisposeAsync()
     {