fix: review fixes for WsReadInfo and WsUpgrade

- WsReadInfo: validate 64-bit frame payload length against maxPayload
  before casting to int (prevents overflow/memory exhaustion)
- WsReadInfo: always send close response per RFC 6455 Section 5.5.1,
  including for empty close frames
- WsUpgrade: restrict no-masking to leaf node connections only (browser
  clients must always mask frames)

src/NATS.Server/WebSocket/WsReadInfo.cs

@@ -176,7 +176,10 @@ public struct WsReadInfo
                     {
                         var (lenBuf, p2) = WsGet(stream, buf, pos, max, 8);
                         pos = p2;
-                        r.Remaining = (int)BinaryPrimitives.ReadUInt64BigEndian(lenBuf);
+                        var len64 = BinaryPrimitives.ReadUInt64BigEndian(lenBuf);
+                        if (len64 > (ulong)maxPayload)
+                            throw new InvalidOperationException($"frame payload length {len64} exceeds max payload {maxPayload}");
+                        r.Remaining = (int)len64;
                         break;
                     }
                 }
@@ -264,11 +267,17 @@ public struct WsReadInfo
                     if (payload.Length > WsConstants.CloseStatusSize)
                         r.CloseBody = Encoding.UTF8.GetString(payload.AsSpan(WsConstants.CloseStatusSize));
                 }
+                // Per RFC 6455 Section 5.5.1, always send a close response
                 if (r.CloseStatus != WsConstants.CloseStatusNoStatusReceived)
                 {
                     var closeMsg = WsFrameWriter.CreateCloseMessage(r.CloseStatus, r.CloseBody ?? "");
                     r.PendingControlFrames.Add(new ControlFrameAction(WsConstants.CloseMessage, closeMsg));
                 }
+                else
+                {
+                    // Empty close frame — respond with empty close
+                    r.PendingControlFrames.Add(new ControlFrameAction(WsConstants.CloseMessage, []));
+                }
                 break;
 
             case WsConstants.PingMessage:

src/NATS.Server/WebSocket/WsUpgrade.cs

@@ -62,8 +62,9 @@ public static class WsUpgrade
                            ext.Contains(WsConstants.PmcExtension, StringComparison.OrdinalIgnoreCase);
             }
 
-            // No-masking support
-            bool noMasking = headers.TryGetValue(WsConstants.NoMaskingHeader, out var nmVal) &&
+            // No-masking support (leaf nodes only — browser clients must always mask)
+            bool noMasking = kind == WsClientKind.Leaf &&
+                             headers.TryGetValue(WsConstants.NoMaskingHeader, out var nmVal) &&
                              string.Equals(nmVal.Trim(), WsConstants.NoMaskingValue, StringComparison.OrdinalIgnoreCase);
 
             // Browser detection