feat: add atomic batch publish engine & versioning support (Tasks 9-10)

- AtomicBatchPublishEngine: stage/commit/rollback semantics for batch publish
- JsVersioning: API level negotiation and stream/consumer metadata
- Fix NormalizeConfig missing AllowAtomicPublish, Metadata, PersistMode copy
- 46 batch publish tests + 67 versioning tests, all passing

tests/NATS.Server.Tests/JetStreamApiFixture.cs

@@ -191,6 +191,60 @@ internal sealed class JetStreamApiFixture : IAsyncDisposable
         return Task.FromResult(new PubAck { ErrorCode = 404 });
     }
 
+    /// <summary>
+    /// Publishes a batch message with the Nats-Batch-Id, Nats-Batch-Sequence (and optionally
+    /// Nats-Batch-Commit) headers simulated via PublishOptions.
+    /// Returns PubAck with ErrorCode set on error, empty BatchId on staged (flow-control), or
+    /// full ack with BatchId+BatchSize on commit.
+    /// </summary>
+    public Task<PubAck> BatchPublishAsync(
+        string subject,
+        string payload,
+        string batchId,
+        ulong batchSeq,
+        string? commitValue = null,
+        string? msgId = null,
+        ulong expectedLastSeq = 0,
+        string? expectedLastMsgId = null)
+    {
+        var options = new PublishOptions
+        {
+            BatchId = batchId,
+            BatchSeq = batchSeq,
+            BatchCommit = commitValue,
+            MsgId = msgId,
+            ExpectedLastSeq = expectedLastSeq,
+            ExpectedLastMsgId = expectedLastMsgId,
+        };
+
+        if (_publisher.TryCaptureWithOptions(subject, Encoding.UTF8.GetBytes(payload), options, out var ack))
+            return Task.FromResult(ack);
+
+        return Task.FromResult(new PubAck { ErrorCode = 404 });
+    }
+
+    public StreamConfig? GetStreamConfig(string streamName)
+    {
+        return _streamManager.TryGet(streamName, out var handle) ? handle.Config : null;
+    }
+
+    public bool UpdateStream(StreamConfig config)
+    {
+        var result = _streamManager.CreateOrUpdate(config);
+        return result.Error == null;
+    }
+
+    public JetStreamApiResponse UpdateStreamWithResult(StreamConfig config)
+    {
+        return _streamManager.CreateOrUpdate(config);
+    }
+
+    /// <summary>
+    /// Exposes the underlying JetStreamPublisher for advanced test scenarios
+    /// (e.g. calling ClearBatches to simulate a leader change).
+    /// </summary>
+    public JetStreamPublisher GetPublisher() => _publisher;
+
     public Task<JetStreamApiResponse> RequestLocalAsync(string subject, string payload)
     {
         return Task.FromResult(_router.Route(subject, Encoding.UTF8.GetBytes(payload)));