fix(nuxt): Chain Nuxt 5 server source maps to the original sources

[s1gr1d]

On Nuxt 5, server stack frames resolve to .nuxt/vite/services/ssr/..., not to the .vue and .ts files. Nitro 3 bundles the output of the ssr build again, but it loads that output without the .map files next to it, so the final maps stop there. The module now adds a load hook to the Nitro build. For each file in <buildDir>/vite/services/ that has a .map, the hook returns the code with its map, and the bundler chains both maps. On Nuxt 3 and 4, this directory does not exist, so the hook does nothing.

This is a workaround. Nuxt has a plugin for this (SourcemapPreserverPlugin), but it adds its load hook only when Nitro does not build as a Vite environment. Its code comment says there is no second bundling pass in that mode, but there is one. When Nuxt fixes this, we can remove the hook.

The issue also asks to keep the Sentry Vite plugin out of the ssr service. No change is needed for that: @nuxt/kit 4.6 scopes addVitePlugin(..., { server: false }) to the client environment, and the existing "one debug ID per file" E2E check covers it.

The assertion in nuxt-5-sourcemaps that the server maps reach app/pages/index.vue is enabled again.

Fixes #25323