feat: implement jetstream pull consumer fetch

tests/NATS.Server.Tests/JetStreamPullConsumerTests.cs

@@ -0,0 +1,15 @@
+namespace NATS.Server.Tests;
+
+public class JetStreamPullConsumerTests
+{
+    [Fact]
+    public async Task Pull_consumer_fetch_returns_available_messages()
+    {
+        await using var fixture = await JetStreamApiFixture.StartWithPullConsumerAsync();
+
+        await fixture.PublishAndGetAckAsync("orders.created", "1");
+        var batch = await fixture.FetchAsync("ORDERS", "PULL", batch: 1);
+
+        batch.Messages.Count.ShouldBe(1);
+    }
+}