feat: add idle heartbeat with pending count headers and flow control stall detection (Gap 3.5)

Heartbeat frames now include Nats-Pending-Messages and Nats-Pending-Bytes
headers populated from the ConsumerHandle. Flow control frames increment
FlowControlPendingCount; AcknowledgeFlowControl() decrements it. IsFlowControlStalled
returns true when pending count reaches MaxFlowControlPending (2).

Go reference: consumer.go:5222 (sendIdleHeartbeat), consumer.go:5495 (sendFlowControl).

src/NATS.Server/JetStream/ConsumerManager.cs

@@ -320,4 +320,11 @@ public sealed record ConsumerHandle(string Stream, ConsumerConfig Config)
     public Queue<PushFrame> PushFrames { get; } = new();
     public AckProcessor AckProcessor { get; } = new();
     public DateTime NextPushDataAvailableAtUtc { get; set; }
+
+    /// <summary>
+    /// Total pending bytes across all unacknowledged messages.
+    /// Included in idle heartbeat headers as Nats-Pending-Bytes.
+    /// Go reference: consumer.go sendIdleHeartbeat.
+    /// </summary>
+    public long PendingBytes { get; set; }
 }