Resolve all high-severity architecture review findings

src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Adapters/RealMxGatewayClient.cs

@@ -216,7 +216,7 @@ public sealed class RealMxGatewayClient : IMxGatewayClient
             // UnsubscribeBulk unadvises AND removes the item, releasing every advice kind.
             _supervisoryAdvised.TryRemove(handle, out _);
             if (_handleToTag.TryRemove(handle, out var tag))
-                _tagToHandle.TryRemove(tag, out _);
+                ((ICollection<KeyValuePair<string, int>>)_tagToHandle).Remove(new(tag, handle));
         }
     }
 
@@ -233,7 +233,7 @@ public sealed class RealMxGatewayClient : IMxGatewayClient
         // so forget any supervisory-advise state for this handle.
         _supervisoryAdvised.TryRemove(handle, out _);
         if (_handleToTag.TryRemove(handle, out var tag))
-            _tagToHandle.TryRemove(tag, out _);
+            ((ICollection<KeyValuePair<string, int>>)_tagToHandle).Remove(new(tag, handle));
     }
 
     /// <inheritdoc />