Fix debug build paths Pages Router support entries (#93529)
## Summary
- Preserve Pages Router support entries during Turbopack
`--debug-build-paths` selective builds
- Include custom Pages Router `404` and `500` entries alongside `_app`,
`_document`, and `_error` when a Pages route is selected
- Add regression coverage for selected Pages routes retaining support
entries in `pages-manifest.json`
## Testing
- `RUSTC="$(rustup which --toolchain nightly-2026-06-24 rustc)" cargo
check -p next-api`
- `RUSTC="$(rustup which --toolchain nightly-2026-06-24 rustc)" pnpm
--dir packages/next-swc build-native`
- `HEADLESS=true pnpm test-start-turbo
test/production/debug-build-path/debug-build-paths.test.ts -t "single
page with pages/ prefix"`
- `HEADLESS=true pnpm test-start-turbo
test/production/debug-build-path/debug-build-paths.test.ts`
Fixes #93515
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