astro integration does not work with astro:i18n prefixDefaultLocale: true

[stefanprobst]

in an astro project, which sets the astro i18n option prefixDefaultLocale to true, i cannot load /keystatic but instead see a 404 page.

not sure if there is something the keystatic astro integration needs to set, or if this is an upstream issue with astro:i18n.

issue repro: https://github.com/stefanprobst/issue-keystatic-astro-i18n-prefix

[emmatown]

This seems like expected behaviour, since /keystatic isn't prefixed with a locale, prefixDefaultLocale says it'll return a 404

[simonswiss]

Collating this in #1080