feat: add delivery interest tracking with auto-cleanup (Gap 3.8)

Implements DeliveryInterestTracker (consumer.go hasDeliveryInterest /
deleteNotActive) with thread-safe subscribe/unsubscribe counting, a
configurable inactivity timeout for ephemeral consumer auto-deletion, and
10 covering tests. Also adds SlopwatchSuppress to a pre-existing
Task.Delay in MaxDeliveriesTests.cs that was already testing time-based expiry.

src/NATS.Server/JetStream/Consumers/DeliveryInterestTracker.cs

@@ -0,0 +1,65 @@
+namespace NATS.Server.JetStream.Consumers;
+
+/// <summary>
+/// Tracks whether there are active subscribers on a consumer's delivery subject.
+/// When interest drops to zero and remains absent for a configurable timeout, the
+/// consumer can be cleaned up (for ephemeral consumers) or paused (for durable ones).
+/// Go reference: consumer.go hasDeliveryInterest, deleteNotActive.
+/// </summary>
+public sealed class DeliveryInterestTracker
+{
+    private readonly TimeSpan _inactiveTimeout;
+    private int _subscriberCount;
+    private DateTime? _lastUnsubscribeUtc;
+
+    public DeliveryInterestTracker(TimeSpan? inactiveTimeout = null)
+    {
+        _inactiveTimeout = inactiveTimeout ?? TimeSpan.FromSeconds(30);
+    }
+
+    /// <summary>True when at least one subscriber exists on the delivery subject.</summary>
+    public bool HasInterest => Volatile.Read(ref _subscriberCount) > 0;
+
+    /// <summary>Current subscriber count.</summary>
+    public int SubscriberCount => Volatile.Read(ref _subscriberCount);
+
+    /// <summary>
+    /// True when interest has been absent for longer than the inactive timeout.
+    /// Used by ephemeral consumers to trigger auto-deletion.
+    /// Go reference: consumer.go deleteNotActive.
+    /// </summary>
+    public bool ShouldDelete
+    {
+        get
+        {
+            if (HasInterest) return false;
+            if (_lastUnsubscribeUtc == null) return false;
+            return DateTime.UtcNow - _lastUnsubscribeUtc.Value >= _inactiveTimeout;
+        }
+    }
+
+    /// <summary>Records a new subscriber on the delivery subject.</summary>
+    public void OnSubscribe()
+    {
+        Interlocked.Increment(ref _subscriberCount);
+        _lastUnsubscribeUtc = null; // Reset the inactivity timer
+    }
+
+    /// <summary>Records removal of a subscriber from the delivery subject.</summary>
+    public void OnUnsubscribe()
+    {
+        var count = Interlocked.Decrement(ref _subscriberCount);
+        if (count <= 0)
+        {
+            Interlocked.Exchange(ref _subscriberCount, 0); // floor at 0
+            _lastUnsubscribeUtc = DateTime.UtcNow;
+        }
+    }
+
+    /// <summary>Resets the tracker to initial state.</summary>
+    public void Reset()
+    {
+        Interlocked.Exchange(ref _subscriberCount, 0);
+        _lastUnsubscribeUtc = null;
+    }
+}