feat: enforce jetstream ack and redelivery semantics

tests/NATS.Server.Tests/JetStreamAckRedeliveryTests.cs

@@ -0,0 +1,17 @@
+namespace NATS.Server.Tests;
+
+public class JetStreamAckRedeliveryTests
+{
+    [Fact]
+    public async Task Unacked_message_is_redelivered_after_ack_wait()
+    {
+        await using var fixture = await JetStreamApiFixture.StartWithAckExplicitConsumerAsync(ackWaitMs: 50);
+        await fixture.PublishAndGetAckAsync("orders.created", "1");
+
+        var first = await fixture.FetchAsync("ORDERS", "PULL", batch: 1);
+        var second = await fixture.FetchAfterDelayAsync("ORDERS", "PULL", delayMs: 75, batch: 1);
+
+        second.Messages.Single().Sequence.ShouldBe(first.Messages.Single().Sequence);
+        second.Messages.Single().Redelivered.ShouldBeTrue();
+    }
+}