feat: execute post-baseline jetstream parity plan

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

@@ -2,22 +2,50 @@ namespace NATS.Server.JetStream.Consumers;
 
 public sealed class AckProcessor
 {
-    private readonly Dictionary<ulong, DateTime> _pending = new();
+    private readonly Dictionary<ulong, PendingState> _pending = new();
 
     public void Register(ulong sequence, int ackWaitMs)
     {
-        _pending[sequence] = DateTime.UtcNow.AddMilliseconds(Math.Max(ackWaitMs, 1));
+        if (_pending.ContainsKey(sequence))
+            return;
+
+        _pending[sequence] = new PendingState
+        {
+            DeadlineUtc = DateTime.UtcNow.AddMilliseconds(Math.Max(ackWaitMs, 1)),
+            Deliveries = 1,
+        };
     }
 
-    public ulong? NextExpired()
+    public bool TryGetExpired(out ulong sequence, out int deliveries)
     {
-        foreach (var (seq, deadline) in _pending)
+        foreach (var (seq, state) in _pending)
         {
-            if (DateTime.UtcNow >= deadline)
-                return seq;
+            if (DateTime.UtcNow >= state.DeadlineUtc)
+            {
+                sequence = seq;
+                deliveries = state.Deliveries;
+                return true;
+            }
         }
 
-        return null;
+        sequence = 0;
+        deliveries = 0;
+        return false;
+    }
+
+    public void ScheduleRedelivery(ulong sequence, int delayMs)
+    {
+        if (!_pending.TryGetValue(sequence, out var state))
+            return;
+
+        state.Deliveries++;
+        state.DeadlineUtc = DateTime.UtcNow.AddMilliseconds(Math.Max(delayMs, 1));
+        _pending[sequence] = state;
+    }
+
+    public void Drop(ulong sequence)
+    {
+        _pending.Remove(sequence);
     }
 
     public bool HasPending => _pending.Count > 0;
@@ -28,4 +56,10 @@ public sealed class AckProcessor
         foreach (var key in _pending.Keys.Where(k => k <= sequence).ToArray())
             _pending.Remove(key);
     }
+
+    private sealed class PendingState
+    {
+        public DateTime DeadlineUtc { get; set; }
+        public int Deliveries { get; set; }
+    }
 }