fix(opcuaclient): #670 review — subscription loss with a live session, stale recovery latch

M1: a subscription that dies while the session lives (outage longer than
the subscription lifetime but shorter than the session timeout) used to
end Healthy with fresh keep-alive stamps and frozen values. PublishError
BadNoSubscription / BadSubscriptionIdInvalid naming a tracked
subscription, or a tracked subscription whose SDK PublishingStopped
persists one more keep-alive window + 5 s across good keep-alives, now
declares it lost: Degraded, outage published once, and nothing advances
LastSuccessfulRead until the session is released, so the #527 watchdog
re-initialises and re-subscribes. Both detectors wire-tested.

M2: in-place recovery is bound to the session whose keep-alive recovered;
one overtaken by a re-initialise no longer re-arms the new session's
outage latch or degrades its health (ReadCoreAsync expectedSession).

L1 CAS health updates; L2 re-check before each recovery publish; L3 skip
CloseSession on a lost session; L4 SDK log forwarding rate-limited
(20/min); L5 per-driver SDK LoggerFactory disposed with the driver.
Wire test asserts the read stamp stays frozen while the server is down.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/SdkTelemetry.cs

@@ -17,19 +17,85 @@ internal static class SdkTelemetry
     /// <summary>Gets the shared, provider-less telemetry context.</summary>
     public static ITelemetryContext Context => Instance.Value;
 
-    /// <summary>Creates a telemetry context whose SDK log output at Warning and above goes to <paramref name="logger"/>.</summary>
+    /// <summary>
+    ///     Creates a telemetry context whose SDK log output at Warning and above goes to <paramref name="logger"/>,
+    ///     rate-limited (a dead upstream makes the SDK warn on every keep-alive and publish for as long as the
+    ///     outage lasts). The context owns a <see cref="ILoggerFactory"/>: release it with <see cref="Release"/>
+    ///     when the driver is disposed — <see cref="DefaultTelemetry"/> is not itself disposable.
+    /// </summary>
     /// <param name="logger">The driver instance's logger.</param>
     /// <returns>The telemetry context.</returns>
     public static ITelemetryContext ForLogger(ILogger logger) =>
         DefaultTelemetry.Create(builder => builder
             .SetMinimumLevel(LogLevel.Warning)
-            .AddProvider(new ForwardingLoggerProvider(logger)));
+            .AddProvider(new ForwardingLoggerProvider(new RateLimitedLogger(logger, MaxEntriesPerWindow, Window))));
 
-    /// <summary>Hands every SDK logger category the one driver logger.</summary>
+    /// <summary>Disposes the logger factory a <see cref="ForLogger"/> context owns. Never the shared one.</summary>
+    /// <param name="context">The context to release.</param>
+    public static void Release(ITelemetryContext context)
+    {
+        if (ReferenceEquals(context, Context)) return;
+        try { (context as TelemetryContextBase)?.LoggerFactory?.Dispose(); }
+        catch { /* best-effort */ }
+    }
+
+    /// <summary>SDK log entries forwarded per <see cref="Window"/> before the rest are dropped and counted.</summary>
+    internal const int MaxEntriesPerWindow = 20;
+
+    /// <summary>The rate-limit window.</summary>
+    internal static readonly TimeSpan Window = TimeSpan.FromMinutes(1);
+
+    /// <summary>Hands every SDK logger category the one (rate-limited) driver logger.</summary>
     private sealed class ForwardingLoggerProvider(ILogger target) : ILoggerProvider
     {
         public ILogger CreateLogger(string categoryName) => target;
 
         public void Dispose() { }
     }
+
+    /// <summary>
+    ///     Forwards at most <c>max</c> entries per window; the first entry of the next window carries a note of how
+    ///     many were suppressed.
+    /// </summary>
+    internal sealed class RateLimitedLogger(ILogger target, int max, TimeSpan window) : ILogger
+    {
+        private readonly object _lock = new();
+        private DateTime _windowStart = DateTime.MinValue;
+        private int _count;
+        private int _suppressed;
+
+        public IDisposable? BeginScope<TState>(TState state) where TState : notnull => target.BeginScope(state);
+
+        public bool IsEnabled(LogLevel logLevel) => target.IsEnabled(logLevel);
+
+        public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception? exception,
+            Func<TState, Exception?, string> formatter)
+        {
+            int suppressedNote;
+            lock (_lock)
+            {
+                var now = DateTime.UtcNow;
+                if (now - _windowStart >= window)
+                {
+                    suppressedNote = _suppressed;
+                    _windowStart = now;
+                    _count = 0;
+                    _suppressed = 0;
+                }
+                else suppressedNote = 0;
+
+                if (_count >= max)
+                {
+                    _suppressed++;
+                    return;
+                }
+                _count++;
+            }
+
+            if (suppressedNote > 0)
+                target.LogWarning("OPC UA SDK: {Suppressed} further log entries were suppressed in the last {Window}.",
+                    suppressedNote, window);
+            target.Log(logLevel, eventId, state, exception, formatter);
+        }
+    }
 }