fix(lmxproxy): wire MxAccess COM subscriptions in SubscriptionManager

SubscriptionManager tracked client-to-tag routing but never called
MxAccessClient.SubscribeAsync to create the actual COM subscriptions,
so OnDataChange never fired. Now creates MxAccess subscriptions for
new tags and disposes them when the last client unsubscribes.

All 17 integration tests pass.

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

lmxproxy/src/ZB.MOM.WW.LmxProxy.Host/Subscriptions/SubscriptionManager.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using System.Threading.Channels;
+using System.Threading.Tasks;
 using Serilog;
 using ZB.MOM.WW.LmxProxy.Host.Domain;
 
@@ -29,6 +30,10 @@ namespace ZB.MOM.WW.LmxProxy.Host.Subscriptions
         private readonly ConcurrentDictionary<string, TagSubscription> _tagSubscriptions
             = new ConcurrentDictionary<string, TagSubscription>(StringComparer.OrdinalIgnoreCase);
 
+        // Tag address -> MxAccess subscription handle (for cleanup when last client unsubscribes)
+        private readonly ConcurrentDictionary<string, IAsyncDisposable> _mxAccessHandles
+            = new ConcurrentDictionary<string, IAsyncDisposable>(StringComparer.OrdinalIgnoreCase);
+
         private readonly ReaderWriterLockSlim _rwLock = new ReaderWriterLockSlim();
 
         public SubscriptionManager(IScadaClient scadaClient, int channelCapacity = 1000,
@@ -58,6 +63,8 @@ namespace ZB.MOM.WW.LmxProxy.Host.Subscriptions
 
             _clientSubscriptions[clientId] = clientSub;
 
+            var newTags = new List<string>();
+
             _rwLock.EnterWriteLock();
             try
             {
@@ -71,6 +78,7 @@ namespace ZB.MOM.WW.LmxProxy.Host.Subscriptions
                     {
                         _tagSubscriptions[address] = new TagSubscription(address,
                             new HashSet<string>(StringComparer.OrdinalIgnoreCase) { clientId });
+                        newTags.Add(address);
                     }
                 }
             }
@@ -79,13 +87,40 @@ namespace ZB.MOM.WW.LmxProxy.Host.Subscriptions
                 _rwLock.ExitWriteLock();
             }
 
+            // Create MxAccess COM subscriptions for newly subscribed tags
+            if (newTags.Count > 0)
+            {
+                _ = CreateMxAccessSubscriptionsAsync(newTags);
+            }
+
             // Register cancellation cleanup
             ct.Register(() => UnsubscribeClient(clientId));
 
-            Log.Information("Client {ClientId} subscribed to {Count} tags", clientId, addressSet.Count);
+            Log.Information("Client {ClientId} subscribed to {Count} tags ({NewCount} new MxAccess subscriptions)",
+                clientId, addressSet.Count, newTags.Count);
             return channel.Reader;
         }
 
+        private async Task CreateMxAccessSubscriptionsAsync(List<string> addresses)
+        {
+            try
+            {
+                var handle = await _scadaClient.SubscribeAsync(
+                    addresses,
+                    (address, vtq) => OnTagValueChanged(address, vtq));
+
+                // Store handle for each address so we can dispose per-tag
+                foreach (var address in addresses)
+                {
+                    _mxAccessHandles[address] = handle;
+                }
+            }
+            catch (Exception ex)
+            {
+                Log.Error(ex, "Failed to create MxAccess subscriptions for {Count} tags", addresses.Count);
+            }
+        }
+
         /// <summary>
         ///     Called from MxAccessClient's OnDataChange handler.
         ///     Fans out the update to all subscribed clients.
@@ -135,6 +170,8 @@ namespace ZB.MOM.WW.LmxProxy.Host.Subscriptions
             if (!_clientSubscriptions.TryRemove(clientId, out var clientSub))
                 return;
 
+            var tagsToDispose = new List<string>();
+
             _rwLock.EnterWriteLock();
             try
             {
@@ -148,6 +185,7 @@ namespace ZB.MOM.WW.LmxProxy.Host.Subscriptions
                         if (tagSub.ClientIds.Count == 0)
                         {
                             _tagSubscriptions.TryRemove(address, out _);
+                            tagsToDispose.Add(address);
                         }
                     }
                 }
@@ -157,6 +195,22 @@ namespace ZB.MOM.WW.LmxProxy.Host.Subscriptions
                 _rwLock.ExitWriteLock();
             }
 
+            // Dispose MxAccess handles for tags with no remaining clients
+            foreach (var address in tagsToDispose)
+            {
+                if (_mxAccessHandles.TryRemove(address, out var handle))
+                {
+                    try
+                    {
+                        handle.DisposeAsync().AsTask().GetAwaiter().GetResult();
+                    }
+                    catch (Exception ex)
+                    {
+                        Log.Warning(ex, "Error disposing MxAccess subscription for {Address}", address);
+                    }
+                }
+            }
+
             // Complete the channel (signals end of stream to the gRPC handler)
             clientSub.Channel.Writer.TryComplete();