feat: implement jetstream push delivery and heartbeat

src/NATS.Server/JetStream/ConsumerManager.cs

@@ -10,6 +10,7 @@ public sealed class ConsumerManager
 {
     private readonly ConcurrentDictionary<(string Stream, string Name), ConsumerHandle> _consumers = new();
     private readonly PullConsumerEngine _pullConsumerEngine = new();
+    private readonly PushConsumerEngine _pushConsumerEngine = new();
 
     public int ConsumerCount => _consumers.Count;
 
@@ -61,10 +62,28 @@ public sealed class ConsumerManager
 
         return await _pullConsumerEngine.FetchAsync(streamHandle, consumer, batch, ct);
     }
+
+    public void OnPublished(string stream, StoredMessage message)
+    {
+        foreach (var handle in _consumers.Values.Where(c => c.Stream == stream && c.Config.Push))
+            _pushConsumerEngine.Enqueue(handle.PushFrames, message, handle.Config);
+    }
+
+    public PushFrame? ReadPushFrame(string stream, string durableName)
+    {
+        if (!_consumers.TryGetValue((stream, durableName), out var consumer))
+            return null;
+
+        if (consumer.PushFrames.Count == 0)
+            return null;
+
+        return consumer.PushFrames.Dequeue();
+    }
 }
 
 public sealed record ConsumerHandle(string Stream, ConsumerConfig Config)
 {
     public ulong NextSequence { get; set; } = 1;
     public Queue<StoredMessage> Pending { get; } = new();
+    public Queue<PushFrame> PushFrames { get; } = new();
 }