refactor(node-fetch-server): add type-safety to socket check in createRequest

[MichaelDeBoey]

The actual code is doing exactly the same, but is introducing type guards for some extra type-safety inside the code itself.

I took inspiration from how React Router handles this check
https://github.com/remix-run/react-router/blob/87912477ce25a501f8cac71c1105dfe3b0bff5ca/packages/react-router-dev/vite/node-adapter.ts#L53-L55

This check was originally introduced in @TrySound's #10199 and resubmitted in remix-run/react-router#13746

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[mjackson]

Thanks, @MichaelDeBoey, but I don't think we need this check.