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/WsConnectionTests.cs

@@ -75,6 +75,42 @@ public class WsConnectionTests
         (written[0] & WsConstants.Rsv1Bit).ShouldBe(0);
     }
 
+    [Fact]
+    public async Task ReadAsync_DecodesMaskedFrame()
+    {
+        var payload = "CONNECT {}\r\n"u8.ToArray();
+        var (header, _) = WsFrameWriter.CreateFrameHeader(
+            useMasking: true, compressed: false,
+            opcode: WsConstants.BinaryMessage, payloadLength: payload.Length);
+        var maskKey = header[^4..];
+        WsFrameWriter.MaskBuf(maskKey, payload);
+
+        var frame = new byte[header.Length + payload.Length];
+        header.CopyTo(frame, 0);
+        payload.CopyTo(frame, header.Length);
+
+        var inner = new MemoryStream(frame);
+        var ws = new WsConnection(inner, compress: false, maskRead: true, maskWrite: false, browser: false, noCompFrag: false);
+
+        var buf = new byte[256];
+        int n = await ws.ReadAsync(buf);
+
+        n.ShouldBe("CONNECT {}\r\n".Length);
+        System.Text.Encoding.ASCII.GetString(buf, 0, n).ShouldBe("CONNECT {}\r\n");
+    }
+
+    [Fact]
+    public async Task ReadAsync_ReturnsZero_OnEndOfStream()
+    {
+        // Empty stream should return 0 (true end of stream)
+        var inner = new MemoryStream([]);
+        var ws = new WsConnection(inner, compress: false, maskRead: false, maskWrite: false, browser: false, noCompFrag: false);
+
+        var buf = new byte[256];
+        int n = await ws.ReadAsync(buf);
+        n.ShouldBe(0);
+    }
+
     private static byte[] BuildUnmaskedFrame(byte[] payload)
     {
         var header = new byte[2];