fix: address code review findings for WebSocket implementation

- Convert WsReadInfo from mutable struct to class (prevents silent copy bugs)
- Add handshake timeout enforcement via CancellationToken in WsUpgrade
- Use buffered reading (512 bytes) in ReadHttpRequestAsync instead of byte-at-a-time
- Add IAsyncDisposable to WsConnection for proper async cleanup
- Simplify redundant mask bit check in WsReadInfo
- Remove unused WsGuid and CompressLastBlock dead code from WsConstants
- Document single-reader assumption on WsConnection read-side state

src/NATS.Server/NatsServer.cs

@@ -568,7 +568,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             }
 
             // HTTP upgrade handshake
-            var upgradeResult = await WsUpgrade.TryUpgradeAsync(stream, stream, _options.WebSocket);
+            var upgradeResult = await WsUpgrade.TryUpgradeAsync(stream, stream, _options.WebSocket, ct);
             if (!upgradeResult.Success)
             {
                 _logger.LogDebug("WebSocket upgrade failed for client {ClientId}", clientId);