Surface historian plugin and alarm-tracking health in the status dashboard so operators can detect misconfiguration and runtime degradation that previously showed as fully healthy

Wraps the 4 HistoryRead overrides and OnAlarmAcknowledge with PerformanceMetrics.BeginOperation, adds alarm counters to LmxNodeManager, publishes a structured HistorianPluginOutcome from HistorianPluginLoader, and extends HealthCheckService with plugin-load, history-read, and alarm-ack-failure degradation rules.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/ZB.MOM.WW.LmxOpcUa.Tests/Status/HealthCheckServiceTests.cs

@@ -105,5 +105,108 @@ namespace ZB.MOM.WW.LmxOpcUa.Tests.Status
             var result = _sut.CheckHealth(ConnectionState.Reconnecting, null);
             result.Status.ShouldBe("Unhealthy");
         }
+
+        /// <summary>
+        ///     Historian enabled but plugin failed to load → Degraded with the plugin error in the message.
+        /// </summary>
+        [Fact]
+        public void HistorianEnabled_PluginLoadFailed_ReturnsDegraded()
+        {
+            var historian = new HistorianStatusInfo
+            {
+                Enabled = true,
+                PluginStatus = "LoadFailed",
+                PluginError = "aahClientManaged.dll could not be loaded"
+            };
+
+            var result = _sut.CheckHealth(ConnectionState.Connected, null, historian);
+
+            result.Status.ShouldBe("Degraded");
+            result.Color.ShouldBe("yellow");
+            result.Message.ShouldContain("LoadFailed");
+            result.Message.ShouldContain("aahClientManaged.dll");
+        }
+
+        /// <summary>
+        ///     Historian disabled is healthy regardless of plugin status string.
+        /// </summary>
+        [Fact]
+        public void HistorianDisabled_ReturnsHealthy()
+        {
+            var historian = new HistorianStatusInfo
+            {
+                Enabled = false,
+                PluginStatus = "Disabled"
+            };
+
+            _sut.CheckHealth(ConnectionState.Connected, null, historian).Status.ShouldBe("Healthy");
+        }
+
+        /// <summary>
+        ///     Historian enabled and plugin loaded is healthy.
+        /// </summary>
+        [Fact]
+        public void HistorianEnabled_PluginLoaded_ReturnsHealthy()
+        {
+            var historian = new HistorianStatusInfo { Enabled = true, PluginStatus = "Loaded" };
+            _sut.CheckHealth(ConnectionState.Connected, null, historian).Status.ShouldBe("Healthy");
+        }
+
+        /// <summary>
+        ///     HistoryRead operations degrade after only 11 samples with &lt;50% success rate
+        ///     (lower threshold than the regular 100-sample rule).
+        /// </summary>
+        [Fact]
+        public void HistoryReadLowSuccessRate_WithLowSampleCount_ReturnsDegraded()
+        {
+            using var metrics = new PerformanceMetrics();
+            for (var i = 0; i < 4; i++)
+                metrics.RecordOperation("HistoryReadRaw", TimeSpan.FromMilliseconds(10));
+            for (var i = 0; i < 8; i++)
+                metrics.RecordOperation("HistoryReadRaw", TimeSpan.FromMilliseconds(10), false);
+
+            var result = _sut.CheckHealth(ConnectionState.Connected, metrics);
+
+            result.Status.ShouldBe("Degraded");
+            result.Message.ShouldContain("HistoryReadRaw");
+        }
+
+        /// <summary>
+        ///     A HistoryRead sample under the 10-sample threshold does not degrade the service.
+        /// </summary>
+        [Fact]
+        public void HistoryReadLowSuccessRate_BelowThreshold_ReturnsHealthy()
+        {
+            using var metrics = new PerformanceMetrics();
+            for (var i = 0; i < 5; i++)
+                metrics.RecordOperation("HistoryReadRaw", TimeSpan.FromMilliseconds(10), false);
+
+            _sut.CheckHealth(ConnectionState.Connected, metrics).Status.ShouldBe("Healthy");
+        }
+
+        /// <summary>
+        ///     Alarm acknowledge write failures are latched — any non-zero count degrades the service.
+        /// </summary>
+        [Fact]
+        public void AlarmAckWriteFailures_AnyCount_ReturnsDegraded()
+        {
+            var alarms = new AlarmStatusInfo { TrackingEnabled = true, AckWriteFailures = 1 };
+
+            var result = _sut.CheckHealth(ConnectionState.Connected, null, null, alarms);
+
+            result.Status.ShouldBe("Degraded");
+            result.Message.ShouldContain("Alarm acknowledge");
+        }
+
+        /// <summary>
+        ///     Alarm tracking disabled ignores any failure count.
+        /// </summary>
+        [Fact]
+        public void AlarmAckWriteFailures_TrackingDisabled_ReturnsHealthy()
+        {
+            var alarms = new AlarmStatusInfo { TrackingEnabled = false, AckWriteFailures = 99 };
+
+            _sut.CheckHealth(ConnectionState.Connected, null, null, alarms).Status.ShouldBe("Healthy");
+        }
     }
 }