fix: handle 408 Requests Pending responses for fetch requests

src/NATS.Client.Core/NatsHeaderParser.cs

@@ -119,6 +119,11 @@ private bool TryParseHeaderLine(ReadOnlySpan<byte> headerLine, NatsHeaders heade
                 headers.Message = NatsHeaders.Messages.MessageSizeExceedsMaxBytes;
                 headers.MessageText = NatsHeaders.MessageMessageSizeExceedsMaxBytesStr;
             }
+            else if (headerLine.SequenceEqual(NatsHeaders.MessageRequestsPendingBytes))
+            {
+                headers.Message = NatsHeaders.Messages.RequestsPending;
+                headers.MessageText = NatsHeaders.MessageRequestsPendingStr;
+            }
             else
             {
                 headers.Message = NatsHeaders.Messages.Text;

src/NATS.Client.Core/NatsHeaders.cs

@@ -27,6 +27,7 @@ public enum Messages
         NoMessages,
         RequestTimeout,
         MessageSizeExceedsMaxBytes,
+        RequestsPending,
     }
 
     // Uses C# compiler's optimization for static byte[] data
@@ -58,6 +59,10 @@ public enum Messages
     internal static ReadOnlySpan<byte> MessageMessageSizeExceedsMaxBytes => new byte[] { 77, 101, 115, 115, 97, 103, 101, 32, 83, 105, 122, 101, 32, 69, 120, 99, 101, 101, 100, 115, 32, 77, 97, 120, 66, 121, 116, 101, 115 };
     internal static readonly string MessageMessageSizeExceedsMaxBytesStr = "Message Size Exceeds MaxBytes";
 
+    // Requests Pending
+    internal static ReadOnlySpan<byte> MessageRequestsPendingBytes => new byte[] { 82, 101, 113, 117, 101, 115, 116, 115, 32, 80, 101, 110, 100, 105, 110, 103 };
+    internal static readonly string MessageRequestsPendingStr = "Requests Pending";
+
     private static readonly string[] EmptyKeys = Array.Empty<string>();
     private static readonly StringValues[] EmptyValues = Array.Empty<StringValues>();
 

src/NATS.Client.Core/NatsSubBase.cs

@@ -22,6 +22,7 @@ public enum NatsSubEndReason
     Cancelled,
     Exception,
     JetStreamError,
+    RequestsPending,
 }
 
 /// <summary>

src/NATS.Client.JetStream/Internal/NatsJSFetch.cs

@@ -220,6 +220,10 @@ protected override async ValueTask ReceiveInternalAsync(
                     {
                         EndSubscription(NatsSubEndReason.NoMsgs);
                     }
+                    else if (headers is { Code: 408, Message: NatsHeaders.Messages.RequestsPending })
+                    {
+                        EndSubscription(NatsSubEndReason.RequestsPending);
+                    }
                     else if (headers is { Code: 408, Message: NatsHeaders.Messages.RequestTimeout })
                     {
                         EndSubscription(NatsSubEndReason.Timeout);