feat: integrate WebSocket accept loop into NatsServer and NatsClient

Add WebSocket listener support to NatsServer alongside the existing TCP
listener. When WebSocketOptions.Port >= 0, the server binds a second
socket, performs HTTP upgrade via WsUpgrade.TryUpgradeAsync, wraps the
connection in WsConnection for transparent frame/deframe, and hands it
to the standard NatsClient pipeline.

Changes:
- NatsClient: add IsWebSocket and WsInfo properties
- NatsServer: add RunWebSocketAcceptLoopAsync and AcceptWebSocketClientAsync,
  WS listener lifecycle in StartAsync/ShutdownAsync/Dispose
- NatsOptions: change WebSocketOptions.Port default from 0 to -1 (disabled)
- WsConnection.ReadAsync: fix premature end-of-stream when ReadFrames
  returns no payloads by looping until data is available
- Add WsIntegration tests (connect, ping, pub/sub over WebSocket)
- Add WsConnection masked frame and end-of-stream unit tests

src/NATS.Server/NatsOptions.cs

@@ -94,7 +94,7 @@ public sealed class NatsOptions
 public sealed class WebSocketOptions
 {
     public string Host { get; set; } = "0.0.0.0";
-    public int Port { get; set; }
+    public int Port { get; set; } = -1;
     public string? Advertise { get; set; }
     public string? NoAuthUser { get; set; }
     public string? JwtCookie { get; set; }