Underscore-prefix colocated page tests so Astro ignores them

The 404, 500, and [author]/index meta tests lived under src/pages/
without the leading underscore. Astro's file router imports every .ts
in src/pages/ during static-path collection, so the prerender server
evaluated these files, their top-level 'vitest' import threw outside
the runner, and the Cloudflare prerender server 500'd the build.

Rename them with a leading underscore (matching the existing [slug]
tests) so Astro skips them as routes while vitest's src/**/*.test.ts
glob still discovers them. See AGENTS.md constraint #8.