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/Historian/HistorianPluginLoaderTests.cs

@@ -0,0 +1,45 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.LmxOpcUa.Host.Configuration;
+using ZB.MOM.WW.LmxOpcUa.Host.Historian;
+
+namespace ZB.MOM.WW.LmxOpcUa.Tests.Historian
+{
+    /// <summary>
+    ///     Verifies the load-outcome state machine of <see cref="HistorianPluginLoader"/>.
+    /// </summary>
+    public class HistorianPluginLoaderTests
+    {
+        /// <summary>
+        ///     MarkDisabled publishes a Disabled outcome so the dashboard can distinguish
+        ///     "feature off" from "load failed."
+        /// </summary>
+        [Fact]
+        public void MarkDisabled_PublishesDisabledOutcome()
+        {
+            HistorianPluginLoader.MarkDisabled();
+
+            HistorianPluginLoader.LastOutcome.Status.ShouldBe(HistorianPluginStatus.Disabled);
+            HistorianPluginLoader.LastOutcome.Error.ShouldBeNull();
+        }
+
+        /// <summary>
+        ///     When the plugin directory is missing, TryLoad reports NotFound — not LoadFailed —
+        ///     and returns null so the server can start with history disabled.
+        /// </summary>
+        [Fact]
+        public void TryLoad_PluginMissing_ReturnsNullWithNotFoundOutcome()
+        {
+            // The test process runs from a bin directory that does not contain a Historian/
+            // subfolder, so TryLoad will take the file-missing branch.
+            var config = new HistorianConfiguration { Enabled = true };
+
+            var result = HistorianPluginLoader.TryLoad(config);
+
+            result.ShouldBeNull();
+            HistorianPluginLoader.LastOutcome.Status.ShouldBe(HistorianPluginStatus.NotFound);
+            HistorianPluginLoader.LastOutcome.PluginPath.ShouldContain("ZB.MOM.WW.LmxOpcUa.Historian.Aveva.dll");
+            HistorianPluginLoader.LastOutcome.Error.ShouldBeNull();
+        }
+    }
+}