Resolve blocking I/O finding and complete Historian lifecycle test coverage

Move subscribe/unsubscribe I/O outside lock(Lock) in SyncAddressSpace to avoid
blocking all OPC UA operations during rebuilds. Replace blocking ReadAsync calls
for alarm priority/description in dispatch loop with cached subscription values.
Extract IHistorianConnectionFactory so EnsureConnected can be tested without the
SDK runtime — adds 5 connection lifecycle tests (failure, timeout, reconnect,
state resilience, dispose-after-failure). All stability review findings and test
coverage gaps are now fully resolved.

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

tests/ZB.MOM.WW.LmxOpcUa.Tests/Helpers/FakeHistorianConnectionFactory.cs

@@ -0,0 +1,49 @@
+using System;
+using ArchestrA;
+using ZB.MOM.WW.LmxOpcUa.Host.Configuration;
+using ZB.MOM.WW.LmxOpcUa.Host.Historian;
+
+namespace ZB.MOM.WW.LmxOpcUa.Tests.Helpers
+{
+    /// <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
+    {
+        /// <summary>
+        ///     When set, <see cref="CreateAndConnect"/> throws this exception.
+        /// </summary>
+        public Exception? ConnectException { get; set; }
+
+        /// <summary>
+        ///     Number of times <see cref="CreateAndConnect"/> has been called.
+        /// </summary>
+        public int ConnectCallCount { get; private set; }
+
+        /// <summary>
+        ///     When set, called on each <see cref="CreateAndConnect"/> to determine behavior.
+        ///     Receives the call count (1-based). Return null to succeed, or throw to fail.
+        /// </summary>
+        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();
+        }
+    }
+}