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/WebSocket/WsConnection.cs

@@ -39,34 +39,37 @@ public sealed class WsConnection : Stream
         if (_readQueue.Count > 0)
             return DrainReadQueue(buffer.Span);
 
-        // Read raw bytes from inner stream
-        var rawBuf = new byte[Math.Max(buffer.Length, 4096)];
-        int bytesRead = await _inner.ReadAsync(rawBuf.AsMemory(), ct);
-        if (bytesRead == 0) return 0;
-
-        // Decode frames
-        var payloads = WsReadInfo.ReadFrames(ref _readInfo, new MemoryStream(rawBuf, 0, bytesRead), bytesRead, maxPayload: 1024 * 1024);
-
-        // Collect control frame responses
-        if (_readInfo.PendingControlFrames.Count > 0)
+        while (true)
         {
-            lock (_writeLock)
-                _pendingControlWrites.AddRange(_readInfo.PendingControlFrames);
-            _readInfo.PendingControlFrames.Clear();
-            // Write pending control frames
-            await FlushControlFramesAsync(ct);
+            // Read raw bytes from inner stream
+            var rawBuf = new byte[Math.Max(buffer.Length, 4096)];
+            int bytesRead = await _inner.ReadAsync(rawBuf.AsMemory(), ct);
+            if (bytesRead == 0) return 0;
+
+            // Decode frames
+            var payloads = WsReadInfo.ReadFrames(ref _readInfo, new MemoryStream(rawBuf, 0, bytesRead), bytesRead, maxPayload: 1024 * 1024);
+
+            // Collect control frame responses
+            if (_readInfo.PendingControlFrames.Count > 0)
+            {
+                lock (_writeLock)
+                    _pendingControlWrites.AddRange(_readInfo.PendingControlFrames);
+                _readInfo.PendingControlFrames.Clear();
+                // Write pending control frames
+                await FlushControlFramesAsync(ct);
+            }
+
+            if (_readInfo.CloseReceived)
+                return 0;
+
+            foreach (var payload in payloads)
+                _readQueue.Enqueue(payload);
+
+            // If no payloads were decoded (e.g. only frame headers were read),
+            // continue reading instead of returning 0 which signals end-of-stream
+            if (_readQueue.Count > 0)
+                return DrainReadQueue(buffer.Span);
         }
-
-        if (_readInfo.CloseReceived)
-            return 0;
-
-        foreach (var payload in payloads)
-            _readQueue.Enqueue(payload);
-
-        if (_readQueue.Count == 0)
-            return 0;
-
-        return DrainReadQueue(buffer.Span);
     }
 
     public override async ValueTask WriteAsync(ReadOnlyMemory<byte> buffer, CancellationToken ct = default)