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

tests/NATS.Server.Tests/WebSocket/WebSocketOptionsTests.cs

@@ -5,10 +5,10 @@ namespace NATS.Server.Tests.WebSocket;
 public class WebSocketOptionsTests
 {
     [Fact]
-    public void DefaultOptions_PortIsZero_Disabled()
+    public void DefaultOptions_PortIsNegativeOne_Disabled()
     {
         var opts = new WebSocketOptions();
-        opts.Port.ShouldBe(0);
+        opts.Port.ShouldBe(-1);
         opts.Host.ShouldBe("0.0.0.0");
         opts.Compression.ShouldBeFalse();
         opts.NoTls.ShouldBeFalse();
@@ -21,6 +21,6 @@ public class WebSocketOptionsTests
     {
         var opts = new NatsOptions();
         opts.WebSocket.ShouldNotBeNull();
-        opts.WebSocket.Port.ShouldBe(0);
+        opts.WebSocket.Port.ShouldBe(-1);
     }
 }