feat(consumer): add WaitingRequestQueue with expiry and batch/maxBytes tracking

Implements a FIFO pull-request queue (WaitingRequestQueue) with per-request
mutable batch countdown and byte budget tracking, plus RemoveExpired cleanup.
Complements the existing priority-based PullRequestWaitQueue for pull consumer delivery.
Go reference: consumer.go waitQueue / processNextMsgRequest (lines 4276-4450).

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

@@ -0,0 +1,65 @@
+namespace NATS.Server.JetStream.Consumers;
+
+/// <summary>
+/// A pull request with mutable batch and byte tracking for delivery fulfillment.
+/// Go reference: consumer.go waitingRequest / processNextMsgRequest.
+/// </summary>
+public sealed record PullRequest(
+    string ReplyTo,
+    int Batch,
+    long MaxBytes,
+    DateTimeOffset Expires,
+    bool NoWait,
+    string? PinId = null)
+{
+    /// <summary>Remaining messages allowed for this request.</summary>
+    public int RemainingBatch { get; private set; } = Batch;
+
+    /// <summary>Remaining bytes allowed for this request (only meaningful when MaxBytes > 0).</summary>
+    public long RemainingBytes { get; private set; } = MaxBytes;
+
+    /// <summary>True when batch or bytes (if set) are exhausted.</summary>
+    public bool IsExhausted => RemainingBatch <= 0 || (MaxBytes > 0 && RemainingBytes <= 0);
+
+    /// <summary>Decrement remaining batch count by one.</summary>
+    public void ConsumeBatch() => RemainingBatch--;
+
+    /// <summary>Subtract delivered bytes from remaining byte budget.</summary>
+    public void ConsumeBytes(long bytes) => RemainingBytes -= bytes;
+}
+
+/// <summary>
+/// FIFO queue of pull requests with expiry support.
+/// Unlike PullRequestWaitQueue (priority-based), this is a simple FIFO with
+/// RemoveExpired cleanup and mutable request tracking.
+/// Go reference: consumer.go waitQueue / processNextMsgRequest.
+/// </summary>
+public sealed class WaitingRequestQueue
+{
+    private readonly LinkedList<PullRequest> _queue = new();
+
+    public int Count => _queue.Count;
+    public bool IsEmpty => _queue.Count == 0;
+
+    public void Enqueue(PullRequest request) => _queue.AddLast(request);
+
+    public PullRequest? TryDequeue()
+    {
+        if (_queue.Count == 0) return null;
+        var first = _queue.First!.Value;
+        _queue.RemoveFirst();
+        return first;
+    }
+
+    public void RemoveExpired(DateTimeOffset now)
+    {
+        var node = _queue.First;
+        while (node != null)
+        {
+            var next = node.Next;
+            if (node.Value.Expires <= now)
+                _queue.Remove(node);
+            node = next;
+        }
+    }
+}