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/NatsClient.cs

@@ -11,6 +11,7 @@ using NATS.Server.Auth;
 using NATS.Server.Protocol;
 using NATS.Server.Subscriptions;
 using NATS.Server.Tls;
+using NATS.Server.WebSocket;
 
 namespace NATS.Server;
 
@@ -79,6 +80,9 @@ public sealed class NatsClient : IDisposable
     private long _rtt;
     public TimeSpan Rtt => new(Interlocked.Read(ref _rtt));
 
+    public bool IsWebSocket { get; set; }
+    public WsUpgradeResult? WsInfo { get; set; }
+
     public TlsConnectionState? TlsState { get; set; }
     public bool InfoAlreadySent { get; set; }