fix: set headerSent flag

[Eomm]

This fix sets the https://nodejs.org/api/http.html#responseheaderssent value for express+fastify

Since Express error middleware chains execute synchronously, a second middleware runs before writableEnded becomes true. It sees res.headersSent === false and calls res.json() again, triggering a second reply.send() → a second onSend chain → a second safeWriteHead → boom.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct