fix(lmxproxy): resolve subscribe/unsubscribe race condition on client reconnect

Three fixes for the SubscriptionManager/MxAccessClient subscription pipeline:

1. Serialize Subscribe and UnsubscribeClient with a SemaphoreSlim gate to prevent
   race where old-session unsubscribe removes new-session COM subscriptions.
   CreateMxAccessSubscriptionsAsync is now awaited instead of fire-and-forget.

2. Fix dual VTQ delivery in MxAccessClient.OnDataChange — each update was delivered
   twice (once via stored callback, once via OnTagValueChanged property). Now uses
   stored callback as the single delivery path.

3. Store pending tag addresses when CreateMxAccessSubscriptionsAsync fails (MxAccess
   down) and retry them on reconnect via NotifyReconnection/RetryPendingSubscriptionsAsync.

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

lmxproxy/tests/ZB.MOM.WW.LmxProxy.Host.Tests/Health/HealthCheckServiceTests.cs

@@ -104,7 +104,7 @@ namespace ZB.MOM.WW.LmxProxy.Host.Tests.Health
             for (int i = 0; i < 101; i++)
             {
                 using var cts = new CancellationTokenSource();
-                sm.Subscribe("client-" + i, new[] { "tag1" }, cts.Token);
+                await sm.SubscribeAsync("client-" + i, new[] { "tag1" }, cts.Token);
             }
 
             var svc = new HealthCheckService(client, sm, pm);