fix(css): keep deprecated name/originalFileName in synthetic assetFileNames call

[ChrisJr404]

Description

getCssAssetDirname calls the user's assetFileNames once with a fake PreRenderedAsset to find out which directory the emitted CSS will land in. The rolldown migration in 47a4d50 dropped the deprecated name and originalFileName (singular) fields from that synthetic call.

Both fields are flagged @deprecated on rolldown's PreRenderedAsset type, but rolldown still populates them when it really emits the asset. User-provided assetFileNames callbacks routinely branch on chunkInfo.name, including the one in the reproduction for #22434:

assetFileNames(chunkInfo) {
  const name = chunkInfo.name ?? ''
  if (name.endsWith('.css')) return 'css/[name]-[hash].[ext]'
  if (name.match(/\.(png|jpe?g|gif|svg|webp)$/)) return 'img/[name]-[hash].[ext]'
  return '[name]-[hash].[ext]'
}

During the real emit chunkInfo.name is main.css and the CSS lands in css/. During the synthetic call from getCssAssetDirname name is undefined, the css branch is skipped, the fallback [name]-[hash].[ext] is returned, path.dirname of that is ., and so url() rewriting computes paths relative to the build root rather than relative to css/. The CSS ends up containing url(./img/foo-*.png) when it should be url(../img/foo-*.png).

This restores name and originalFileName on the synthetic call so it matches what user code sees during the real emit. The string and undefined (no callback) branches were already correct since they derive the dirname from the pattern itself.

Verification

Reproduced against https://github.com/karlvr/vite-relative-base-bug with vite 8.0.12: emits url(./img/foo-*.png). With this patch the same build emits url(../img/foo-*.png), matching vite 7.

Added a regression test in build.spec.ts that runs a build() programmatically with base: './' and a function assetFileNames that splits CSS into css/ and images into img/. The test asserts the emitted CSS contains url(../img/foo-*.png).

• pnpm test-unit in packages/vite (747 passed, 4 skipped).
• VITE_TEST_BUILD=1 pnpm exec vitest run playground/assets/__tests__/relative-base/ (34 passed).
• VITE_TEST_BUILD=1 pnpm exec vitest run playground/css/__tests__/ (107 passed, 3 skipped).

What is the purpose of this pull request?

• Documentation update
• Bug fix
• New Feature
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

Closes #22434