feat: add cluster-aware pending request tracking for pull consumers (Gap 3.14)

Adds ProposeWaitingRequest, RegisterClusterPending, RemoveClusterPending,
GetClusterPendingRequests, and ClusterPendingCount to PullConsumerEngine,
backed by a ConcurrentDictionary keyed by reply subject. Includes 10 xUnit
tests covering quorum checks, pending tracking, and concurrent access patterns.

src/NATS.Server/JetStream/Consumers/PullConsumerEngine.cs

@@ -1,3 +1,5 @@
+using System.Collections.Concurrent;
+using NATS.Server.JetStream.Cluster;
 using NATS.Server.JetStream.Storage;
 using NATS.Server.JetStream.Models;
 using NATS.Server.Subscriptions;
@@ -93,6 +95,60 @@ public sealed class CompiledFilter
 
 public sealed class PullConsumerEngine
 {
+    // Go: consumer.go — cluster-wide pending pull request tracking keyed by reply subject.
+    // Reference: golang/nats-server/server/consumer.go waitingRequestsPending / proposeWaitingRequest
+    private readonly ConcurrentDictionary<string, PullWaitingRequest> _clusterPending =
+        new(StringComparer.Ordinal);
+
+    /// <summary>
+    /// Number of pending pull requests currently tracked across the cluster.
+    /// Go reference: consumer.go — cluster pending count (waitingRequestsPending).
+    /// </summary>
+    public int ClusterPendingCount => _clusterPending.Count;
+
+    /// <summary>
+    /// Proposes a waiting pull request through the consumer's RAFT group.
+    /// Returns true if quorum is available and the request was registered; false otherwise.
+    /// Go reference: consumer.go proposeWaitingRequest — propose via consumer RAFT group.
+    /// </summary>
+    public bool ProposeWaitingRequest(PullWaitingRequest request, RaftGroup group)
+    {
+        if (!group.HasQuorum(group.Peers.Count))
+            return false;
+
+        var replyKey = request.Reply ?? string.Empty;
+        _clusterPending[replyKey] = request;
+        return true;
+    }
+
+    /// <summary>
+    /// Registers a pull request in the cluster pending tracker, keyed by reply subject.
+    /// Go reference: consumer.go — cluster pending registration on proposal acceptance.
+    /// </summary>
+    public void RegisterClusterPending(PullWaitingRequest request)
+    {
+        var replyKey = request.Reply ?? string.Empty;
+        _clusterPending[replyKey] = request;
+    }
+
+    /// <summary>
+    /// Removes and returns a pending pull request by its reply subject.
+    /// Returns null if no request is registered for that reply subject.
+    /// Go reference: consumer.go — cluster pending removal on fulfillment or expiry.
+    /// </summary>
+    public PullWaitingRequest? RemoveClusterPending(string replySubject)
+    {
+        _clusterPending.TryRemove(replySubject, out var request);
+        return request;
+    }
+
+    /// <summary>
+    /// Returns all currently pending pull requests tracked across the cluster.
+    /// Go reference: consumer.go — enumerate waitingRequestsPending for expiry sweep.
+    /// </summary>
+    public IReadOnlyCollection<PullWaitingRequest> GetClusterPendingRequests()
+        => _clusterPending.Values.ToArray();
+
     public async ValueTask<PullFetchBatch> FetchAsync(StreamHandle stream, ConsumerHandle consumer, int batch, CancellationToken ct)
         => await FetchAsync(stream, consumer, new PullFetchRequest { Batch = batch }, ct);