fix(dcl): discard in-flight subscribe results for unsubscribed tags; release the orphaned handle

tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Actors/FakeBatchDataConnection.cs

@@ -35,6 +35,13 @@ public sealed class FakeBatchDataConnection
     public Func<Exception>? BatchSubscribeThrows;
     /// <summary>When true, bulk reads never return until the caller's token cancels.</summary>
     public bool HangReads;
+    /// <summary>
+    /// When set, <see cref="SubscribeBatchAsync"/> records the call and then parks until this
+    /// task completes, before producing its result rows. Lets a test hold a subscribe batch
+    /// in flight while it drives other messages into the actor (e.g. an unsubscribe that
+    /// races the completion).
+    /// </summary>
+    public Task? SubscribeGate;
     /// <summary>Value returned for every readable tag.</summary>
     public object? SeedValue = 42;
 
@@ -67,7 +74,7 @@ public sealed class FakeBatchDataConnection
     }
 
     /// <inheritdoc />
-    public Task<IReadOnlyList<TagSubscribeResult>> SubscribeBatchAsync(
+    public async Task<IReadOnlyList<TagSubscribeResult>> SubscribeBatchAsync(
         IReadOnlyList<string> tagPaths, SubscriptionCallback callback, CancellationToken cancellationToken = default)
     {
         SubscribeBatches.Enqueue(tagPaths.ToList());
@@ -77,12 +84,15 @@ public sealed class FakeBatchDataConnection
         if (BatchSubscribeThrows is { } factory)
             throw factory();
 
+        if (SubscribeGate is { } gate)
+            await gate;
+
         IReadOnlyList<TagSubscribeResult> rows = tagPaths
             .Select(t => FailingTags.Contains(t)
                 ? new TagSubscribeResult(t, false, null, "node not found")
                 : new TagSubscribeResult(t, true, $"sub-{Interlocked.Increment(ref _nextId)}", null))
             .ToList();
-        return Task.FromResult(rows);
+        return rows;
     }
 
     /// <inheritdoc />