Extract historian into a runtime-loaded plugin so hosts without the Wonderware SDK can run with Historian.Enabled=false

The aahClientManaged SDK is now isolated in ZB.MOM.WW.LmxOpcUa.Historian.Aveva and loaded via HistorianPluginLoader from a Historian/ subfolder only when enabled, removing the SDK from Host's compile-time and deploy-time surface.

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

tests/ZB.MOM.WW.LmxOpcUa.Historian.Aveva.Tests/FakeHistorianConnectionFactory.cs

@@ -0,0 +1,39 @@
+using System;
+using ArchestrA;
+using ZB.MOM.WW.LmxOpcUa.Historian.Aveva;
+using ZB.MOM.WW.LmxOpcUa.Host.Configuration;
+
+namespace ZB.MOM.WW.LmxOpcUa.Historian.Aveva.Tests
+{
+    /// <summary>
+    ///     Fake Historian connection factory for tests. Controls whether connections
+    ///     succeed, fail, or timeout without requiring the real Historian SDK runtime.
+    /// </summary>
+    internal sealed class FakeHistorianConnectionFactory : IHistorianConnectionFactory
+    {
+        public Exception? ConnectException { get; set; }
+
+        public int ConnectCallCount { get; private set; }
+
+        public Action<int>? OnConnect { get; set; }
+
+        public HistorianAccess CreateAndConnect(HistorianConfiguration config, HistorianConnectionType type)
+        {
+            ConnectCallCount++;
+
+            if (OnConnect != null)
+            {
+                OnConnect(ConnectCallCount);
+            }
+            else if (ConnectException != null)
+            {
+                throw ConnectException;
+            }
+
+            // Return a HistorianAccess that is not actually connected.
+            // ReadRawAsync etc. will fail when they try to use it, which exercises
+            // the HandleConnectionError → reconnect path.
+            return new HistorianAccess();
+        }
+    }
+}