feat: complete remaining jetstream parity implementation plan

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

@@ -21,4 +21,11 @@ public sealed class AckProcessor
     }
 
     public bool HasPending => _pending.Count > 0;
+    public int PendingCount => _pending.Count;
+
+    public void AckAll(ulong sequence)
+    {
+        foreach (var key in _pending.Keys.Where(k => k <= sequence).ToArray())
+            _pending.Remove(key);
+    }
 }