fix(data-connection-layer): resolve DataConnectionLayer-002/003/004/005 — Resume supervision, concurrent dicts, subscribe-failure classification, write timeout

tests/ScadaLink.DataConnectionLayer.Tests/OpcUaDataConnectionTests.cs

@@ -6,6 +6,37 @@ using ScadaLink.DataConnectionLayer.Adapters;
 
 namespace ScadaLink.DataConnectionLayer.Tests;
 
+/// <summary>
+/// DataConnectionLayer-003: structural regression guard. RealOpcUaClient's
+/// monitored-item / callback maps are read from the OPC UA SDK's publish threads
+/// concurrently with subscribe/disconnect mutations on other threads. They must be
+/// concurrent collections, not plain Dictionary. This is verified structurally
+/// because RealOpcUaClient wraps concrete OPC Foundation SDK types and cannot be
+/// exercised without a live OPC UA server.
+/// </summary>
+public class RealOpcUaClientThreadSafetyTests
+{
+    [Theory]
+    [InlineData("_callbacks")]
+    [InlineData("_monitoredItems")]
+    public void DCL003_SharedDictionaryFields_AreConcurrentCollections(string fieldName)
+    {
+        var field = typeof(RealOpcUaClient)
+            .GetField(fieldName,
+                System.Reflection.BindingFlags.Instance |
+                System.Reflection.BindingFlags.NonPublic);
+
+        Assert.NotNull(field);
+
+        var fieldType = field!.FieldType;
+        Assert.True(
+            fieldType.IsGenericType &&
+            fieldType.GetGenericTypeDefinition() == typeof(System.Collections.Concurrent.ConcurrentDictionary<,>),
+            $"RealOpcUaClient.{fieldName} must be a ConcurrentDictionary<,> for thread safety, " +
+            $"but was {fieldType.Name}.");
+    }
+}
+
 /// <summary>
 /// WP-7: Tests for OPC UA adapter.
 /// </summary>