fix(prerendering): never prerender custom 500 routes

[lilnasy]

Changes

• Closes Prerended custom 500 page does not include imported stylesheets #8005
• Adds a check whether the route being prerendered is a custom 404 or 500 one.

Future Work

• Custom 500 routes don't work in dev mode at all.

Testing

Pending

Docs

Does not affect usage

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[lilnasy]

To be clear, these routes are currently rendered just-in-time, or don't work at all (at least, with the node adapter.)

This change prevents the manifest corruption that occurs when prerendering is attempted.

[matthewp]

sgtm

[lilnasy]

Astro core doesn't support prerendering 404.astro, it is prerendered into 404.html but the router renders it just-in-time anyway. However, the netlify adapter is able to achieve the effect by using _redirects, so it would be a regression to consider it ineligible for prerendering.

Only considering 500.astro ineligible still solves the immediate issue.

I think actually prerendering using the prerendered 404 and 500 routes can be revisited after the "pipeline" refactors.

[lilnasy]

There is a better way.

Closing in favor of #8070