[Flight] Add more DoS mitigations to Flight Reply, and harden Flight (#35632)

This fixes security vulnerabilities in Server Functions.

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Co-authored-by: Sebastian Markbåge <sebastian@calyptus.eu>
Co-authored-by: Josh Story <josh.c.story@gmail.com>
Co-authored-by: Janka Uryga <lolzatu2@gmail.com>
Co-authored-by: Sebastian Sebbie Silbermann <sebastian.silbermann@vercel.com>