chore: ensure @vitejs/ scope for private packages names

[hi-ogawa]

Description

This PR fixes 5 test packages which were not following @vitejs/ scope convention. When violated, CI error looks like this https://github.com/vitejs/vite/actions/runs/17849457324/job/50755068346#step:11:19

Error: Found private packages without @vitejs/ prefix:
  - docs (/home/runner/work/vite/vite/docs)
  - dep (/home/runner/work/vite/vite/playground/css/pkg-dep)
  - blue-app (/home/runner/work/vite/vite/playground/css/postcss-caching/blue-app)
  - green-app (/home/runner/work/vite/vite/playground/css/postcss-caching/green-app)
  - nested-external-cjs (/home/runner/work/vite/vite/playground/ssr-deps/nested-external-cjs)

This seems to help not getting flagged security issue etc. when there are real packages exist and getting compromised.

[bluwy]

I'm not so sure we need a CI step to catch this, we could manually ensure to use scope going forward. The rest of the changes looks fine to me though.

[hi-ogawa]

Right, I'm fine either way. AI just wrote this job and that was it 😀

[sapphi-red]

I think we can go without the check in the CI.

[sapphi-red]

Thanks!