feat: implement jetstream pull consumer fetch

tests/NATS.Server.Tests/JetStreamApiFixture.cs

@@ -1,6 +1,7 @@
 using System.Text;
 using NATS.Server.JetStream;
 using NATS.Server.JetStream.Api;
+using NATS.Server.JetStream.Consumers;
 using NATS.Server.JetStream.Models;
 using NATS.Server.JetStream.Publish;
 
@@ -38,6 +39,13 @@ internal sealed class JetStreamApiFixture : IAsyncDisposable
         return fixture;
     }
 
+    public static async Task<JetStreamApiFixture> StartWithPullConsumerAsync()
+    {
+        var fixture = await StartWithStreamAsync("ORDERS", "orders.*");
+        _ = await fixture.CreateConsumerAsync("ORDERS", "PULL", "orders.created");
+        return fixture;
+    }
+
     public Task<PubAck> PublishAndGetAckAsync(string subject, string payload, string? msgId = null, bool expectError = false)
     {
         if (_publisher.TryCapture(subject, Encoding.UTF8.GetBytes(payload), msgId, out var ack))
@@ -71,5 +79,10 @@ internal sealed class JetStreamApiFixture : IAsyncDisposable
         return response.ConsumerInfo ?? throw new InvalidOperationException("Consumer not found.");
     }
 
+    public Task<PullFetchBatch> FetchAsync(string stream, string durableName, int batch)
+    {
+        return _consumerManager.FetchAsync(stream, durableName, batch, _streamManager, default).AsTask();
+    }
+
     public ValueTask DisposeAsync() => ValueTask.CompletedTask;
 }