feat: add MaxPending, WriteDeadline options and error constants

src/NATS.Server/Monitoring/VarzHandler.cs

@@ -66,6 +66,8 @@ public sealed class VarzHandler : IDisposable
                 MaxConnections = _options.MaxConnections,
                 MaxPayload = _options.MaxPayload,
                 MaxControlLine = _options.MaxControlLine,
+                MaxPending = _options.MaxPending,
+                WriteDeadline = (long)_options.WriteDeadline.TotalNanoseconds,
                 MaxPingsOut = _options.MaxPingsOut,
                 PingInterval = (long)_options.PingInterval.TotalNanoseconds,
                 Start = _server.StartTime,

src/NATS.Server/NatsOptions.cs

@@ -11,6 +11,8 @@ public sealed class NatsOptions
     public int MaxPayload { get; set; } = 1024 * 1024;
     public int MaxControlLine { get; set; } = 4096;
     public int MaxConnections { get; set; } = 65536;
+    public long MaxPending { get; set; } = 64 * 1024 * 1024; // 64MB, matching Go MAX_PENDING_SIZE
+    public TimeSpan WriteDeadline { get; set; } = TimeSpan.FromSeconds(10);
     public TimeSpan PingInterval { get; set; } = TimeSpan.FromMinutes(2);
     public int MaxPingsOut { get; set; } = 2;
 

src/NATS.Server/Protocol/NatsProtocol.cs

@@ -6,6 +6,7 @@ public static class NatsProtocol
 {
     public const int MaxControlLineSize = 4096;
     public const int MaxPayloadSize = 1024 * 1024; // 1MB
+    public const long MaxPendingSize = 64 * 1024 * 1024; // 64MB default max pending
     public const int DefaultPort = 4222;
     public const string Version = "0.1.0";
     public const int ProtoVersion = 1;
@@ -30,6 +31,8 @@ public static class NatsProtocol
     public const string ErrAuthTimeout = "Authentication Timeout";
     public const string ErrPermissionsPublish = "Permissions Violation for Publish";
     public const string ErrPermissionsSubscribe = "Permissions Violation for Subscription";
+    public const string ErrSlowConsumer = "Slow Consumer";
+    public const string ErrNoRespondersRequiresHeaders = "No Responders Requires Headers Support";
 }
 
 public sealed class ServerInfo