Auto: ablegacy-12 — auto-demote on comm failure

Closes #255

tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/FakeAbLegacyTag.cs

@@ -84,7 +84,14 @@ internal class FakeAbLegacyTag : IAbLegacyTagRuntime
 
 internal sealed class FakeAbLegacyTagFactory : IAbLegacyTagFactory
 {
-    public Dictionary<string, FakeAbLegacyTag> Tags { get; } = new(StringComparer.OrdinalIgnoreCase);
+    // PR ablegacy-12 / #255 — switched from plain Dictionary to ConcurrentDictionary so
+    // the read path (test thread) and the probe loop (background Task) can both call
+    // Create without corrupting the dict. Pre-PR-12 the race existed but only tipped
+    // a few percent of test runs into KeyNotFoundException; PR-12's added
+    // Interlocked.Exchange writes shifted timing enough to make it deterministic-flaky
+    // (~60%).
+    public System.Collections.Concurrent.ConcurrentDictionary<string, FakeAbLegacyTag> Tags { get; } =
+        new(StringComparer.OrdinalIgnoreCase);
     public Func<AbLegacyTagCreateParams, FakeAbLegacyTag>? Customise { get; set; }
 
     public IAbLegacyTagRuntime Create(AbLegacyTagCreateParams p)