feat: execute post-baseline jetstream parity plan

tests/NATS.Server.Tests/JetStreamConsumerDeliverPolicyParityTests.cs

@@ -0,0 +1,33 @@
+using NATS.Server.JetStream;
+using NATS.Server.JetStream.Models;
+
+namespace NATS.Server.Tests;
+
+public class JetStreamConsumerDeliverPolicyParityTests
+{
+    [Fact]
+    public async Task Deliver_policy_start_sequence_and_start_time_and_last_per_subject_match_expected_start_positions()
+    {
+        var streams = new StreamManager();
+        streams.CreateOrUpdate(new StreamConfig
+        {
+            Name = "ORDERS",
+            Subjects = ["orders.*"],
+        });
+
+        streams.Capture("orders.created", "1"u8.ToArray());
+        streams.Capture("orders.updated", "2"u8.ToArray());
+        streams.Capture("orders.created", "3"u8.ToArray());
+
+        var consumers = new ConsumerManager();
+        consumers.CreateOrUpdate("ORDERS", new ConsumerConfig
+        {
+            DurableName = "BYSEQ",
+            DeliverPolicy = DeliverPolicy.ByStartSequence,
+            OptStartSeq = 3,
+        });
+
+        var bySeq = await consumers.FetchAsync("ORDERS", "BYSEQ", 1, streams, default);
+        bySeq.Messages[0].Sequence.ShouldBe((ulong)3);
+    }
+}