fix: preserve manifest metadata for same-basename CSS entries

[raashish1601]

Summary

• key emitted CSS entry metadata by the original entry path when available instead of the lossy chunk basename
• reuse the same key when replacing pure CSS entry placeholders so manifest bookkeeping resolves the right emitted asset
• add a build regression for two CSS entry points with the same basename in different directories

Fixes #22013.

Testing

• corepack pnpm exec vitest run packages/vite/src/node/__tests__/build.spec.ts -t "manifest keeps same-basename css entries independent"
• corepack pnpm exec eslint packages/vite/src/node/plugins/css.ts packages/vite/src/node/__tests__/build.spec.ts
• git diff --check

[sapphi-red]

Thanks, but I think #22039's approach is better