feat: execute post-baseline jetstream parity plan

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

@@ -7,20 +7,41 @@ public sealed class PushConsumerEngine
 {
     public void Enqueue(ConsumerHandle consumer, StoredMessage message)
     {
+        var availableAtUtc = DateTime.UtcNow;
+        if (consumer.Config.RateLimitBps > 0)
+        {
+            if (consumer.NextPushDataAvailableAtUtc > availableAtUtc)
+                availableAtUtc = consumer.NextPushDataAvailableAtUtc;
+
+            var delayMs = (long)Math.Ceiling((double)message.Payload.Length * 1000 / consumer.Config.RateLimitBps);
+            consumer.NextPushDataAvailableAtUtc = availableAtUtc.AddMilliseconds(Math.Max(delayMs, 1));
+        }
+
         consumer.PushFrames.Enqueue(new PushFrame
         {
             IsData = true,
             Message = message,
+            AvailableAtUtc = availableAtUtc,
         });
 
         if (consumer.Config.AckPolicy is AckPolicy.Explicit or AckPolicy.All)
             consumer.AckProcessor.Register(message.Sequence, consumer.Config.AckWaitMs);
 
+        if (consumer.Config.FlowControl)
+        {
+            consumer.PushFrames.Enqueue(new PushFrame
+            {
+                IsFlowControl = true,
+                AvailableAtUtc = availableAtUtc,
+            });
+        }
+
         if (consumer.Config.HeartbeatMs > 0)
         {
             consumer.PushFrames.Enqueue(new PushFrame
             {
                 IsHeartbeat = true,
+                AvailableAtUtc = availableAtUtc,
             });
         }
     }
@@ -29,6 +50,8 @@ public sealed class PushConsumerEngine
 public sealed class PushFrame
 {
     public bool IsData { get; init; }
+    public bool IsFlowControl { get; init; }
     public bool IsHeartbeat { get; init; }
     public StoredMessage? Message { get; init; }
+    public DateTime AvailableAtUtc { get; init; } = DateTime.UtcNow;
 }