fix(lmxproxy): await COM subscription creation to fix Subscribe flakiness

SubscriptionManager.Subscribe was fire-and-forgetting the MxAccess COM
subscription creation. The initial OnDataChange callback could fire
before the subscription was established, losing the first (and possibly
only) value update. Changed to async SubscribeAsync that awaits
CreateMxAccessSubscriptionsAsync before returning the channel reader.

Subscribe_ReceivesUpdates now passes 5/5 consecutive runs.

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);