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

Closes #255

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

@@ -173,7 +173,9 @@ public sealed class AbLegacyDiagnosticsTests
         var diagVars = builder.Variables
             .Where(v => v.Info.FullName.StartsWith(AbLegacyDiagnosticTags.DiagnosticsFolderPrefix))
             .ToList();
-        diagVars.Count.ShouldBe(14); // 7 names × 2 devices
+        // PR ablegacy-12 / #255 — DemoteCount + LastDemotedUtc bring the canonical
+        // count to 9 names per device (was 7 in PR ablegacy-10).
+        diagVars.Count.ShouldBe(AbLegacyDiagnosticTags.DiagnosticTagNames.Count * 2);
         diagVars.ShouldAllBe(v => v.Info.SecurityClass == SecurityClassification.ViewOnly);
     }