fix: handle socket errors during upgrade hooks

[Dansatch]

Fixes #380

A client disconnecting while an asynchronous upgrade hook was still running could crash the whole server.

This change handles socket errors during that short window, then lets ws take over error handling once the upgrade begins.

A regression test that reproduces the disconnect and confirms the server stays running has been added.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added — not required for this internal bug fix
• commit message and code follows the [Developer's Certification of Origin](https://github.com/fastify/.github/blob/main/CONTRIBUTING.md#developers-certificate-of-origin-11) and the [Code of conduct](https://github.com/fastify/.github/blob/main/CODE_OF_CONDUCT.md)

[Tony133]

lgtm