feat: harden jetstream consumer state machine parity

tests/NATS.Server.Tests/JetStream/JetStreamConsumerStateMachineStrictParityTests.cs

@@ -0,0 +1,42 @@
+using NATS.Server.JetStream;
+using NATS.Server.JetStream.Models;
+
+namespace NATS.Server.Tests.JetStream;
+
+public class JetStreamConsumerStateMachineStrictParityTests
+{
+    [Fact]
+    public async Task Ack_redelivery_backoff_and_replay_timing_follow_monotonic_consumer_state_machine_rules()
+    {
+        var streams = new StreamManager();
+        var consumers = new ConsumerManager();
+
+        streams.CreateOrUpdate(new StreamConfig
+        {
+            Name = "ORDERS_SM",
+            Subjects = ["orders.sm"],
+            Retention = RetentionPolicy.Limits,
+            MaxMsgs = 32,
+        }).Error.ShouldBeNull();
+
+        consumers.CreateOrUpdate("ORDERS_SM", new ConsumerConfig
+        {
+            DurableName = "D1",
+            AckPolicy = AckPolicy.Explicit,
+            AckWaitMs = 1,
+            MaxDeliver = 1,
+            BackOffMs = [1],
+        }).Error.ShouldBeNull();
+
+        streams.Capture("orders.sm", "x"u8.ToArray());
+
+        var first = await consumers.FetchAsync("ORDERS_SM", "D1", 1, streams, default);
+        first.Messages.Count.ShouldBe(1);
+
+        await Task.Delay(5);
+        var second = await consumers.FetchAsync("ORDERS_SM", "D1", 1, streams, default);
+
+        // MaxDeliver=1 means the initial delivery is the only allowed delivery.
+        second.Messages.Count.ShouldBe(0);
+    }
+}