feat: execute post-baseline jetstream parity plan

tests/NATS.Server.Tests/JetStreamConsumerBackoffParityTests.cs

@@ -0,0 +1,41 @@
+using NATS.Server.JetStream;
+using NATS.Server.JetStream.Models;
+
+namespace NATS.Server.Tests;
+
+public class JetStreamConsumerBackoffParityTests
+{
+    [Fact]
+    public async Task Redelivery_honors_backoff_schedule_and_stops_after_max_deliver()
+    {
+        var streams = new StreamManager();
+        streams.CreateOrUpdate(new StreamConfig
+        {
+            Name = "ORDERS",
+            Subjects = ["orders.*"],
+        });
+
+        streams.Capture("orders.created", "x"u8.ToArray());
+
+        var consumers = new ConsumerManager();
+        consumers.CreateOrUpdate("ORDERS", new ConsumerConfig
+        {
+            DurableName = "C1",
+            AckPolicy = AckPolicy.Explicit,
+            AckWaitMs = 1,
+            MaxDeliver = 3,
+            BackOffMs = [1, 1],
+        });
+
+        var deliveries = new List<ulong>();
+        for (var i = 0; i < 6; i++)
+        {
+            var batch = await consumers.FetchAsync("ORDERS", "C1", 1, streams, default);
+            if (batch.Messages.Count > 0 && batch.Messages[0].Redelivered)
+                deliveries.Add(batch.Messages[0].Sequence);
+            await Task.Delay(2);
+        }
+
+        deliveries.Count.ShouldBe(3);
+    }
+}