fix: Allow new Vite v8 outputOptions callback shape

[raybrownco]

Description 📖

Fix a build issue related to Vite 8's handling of the assetFileNames callback, which is manually typed in the outputFileName() function.

Background 📜

Vite v8 (which uses Rolldown) changed the object shape passed to the assetFileNames callback.

In Vite v7 (which uses Rollup), the callback received a PreRenderedAsset object with a singular name: string | undefined field.

In Vite v8, Vite's internal vite:css-post plugin calls the same assetFileNames function with a plural names: string[] field instead. This matches the Rolldown PreRenderedAsset interface. See: https://github.com/rolldown/rolldown/blob/a71934bf/packages/rolldown/src/options/output-options.ts#L66-L77

The Fix 🔨

First, we define a PreRenderedAsset interface that handles both shapes:

export interface PreRenderedAsset {
  name?: string // Vite v7, deprecated
  names?: string[] // Vite v8
  // ...
}

Then, determine which value to use within the outputFileName() function:

const outputFileName = (ext: string) => (asset: PreRenderedAsset) => {
    // Vite v8 uses `names`, earlier versions use `name`
    const resolvedName = asset.names?.[0] ?? asset.name ?? '[name]'
    const shortName = basename(resolvedName).split('.')[0]
    return posix.join(assetsDir, `${shortName}-[hash].${ext}`)
  }

Screenshots 📷

A Rails project with vite_rails (3.0.19) and vite_ruby (3.9.2) began to fail after updating vite from 7.3.2 to 8.0.10:

[raybrownco]

@ElMassimo Ready for your review: this PR fixes a bug that we ran into when attempting to upgrade Vite from v7 to v8 in one of our Rails projects. Thanks in advance for your consideration! Let me know if you object to any of these changes - I'm more than happy to accept feedback.