feat: complete remaining jetstream parity implementation plan

tests/NATS.Server.Tests/JetStreamConsumerNextApiTests.cs

@@ -0,0 +1,15 @@
+namespace NATS.Server.Tests;
+
+public class JetStreamConsumerNextApiTests
+{
+    [Fact]
+    public async Task Consumer_msg_next_respects_batch_request()
+    {
+        await using var fx = await JetStreamApiFixture.StartWithPullConsumerAsync();
+        _ = await fx.PublishAndGetAckAsync("orders.created", "1");
+
+        var next = await fx.RequestLocalAsync("$JS.API.CONSUMER.MSG.NEXT.ORDERS.PULL", "{\"batch\":1}");
+        next.PullBatch.ShouldNotBeNull();
+        next.PullBatch!.Messages.Count.ShouldBe(1);
+    }
+}