* fix(css): pass sassOptions from next.config to Vite preprocessor
Next.js threads `sassOptions` from `next.config.js` into sass-loader,
destructuring `prependData` / `additionalData` / `implementation` and
forwarding the rest (e.g. `includePaths`, `loadPaths`) as Sass options.
vinext was dropping this entirely, so SCSS fixtures that rely on
`additionalData` to define variables, `includePaths`/`loadPaths` to
resolve partials, or a custom `implementation` all failed to build
with `Undefined variable` / `Can't find stylesheet to import`.
Resolve `sassOptions` from next.config into `ResolvedNextConfig`, then
translate it onto Vite's `css.preprocessorOptions.scss` (and `.sass`)
shape:
- prependData precedence matches Next.js (prependData || additionalData)
- legacy includePaths is merged into modern loadPaths (Vite uses
modern Sass, which dropped includePaths)
- implementation and any remaining Sass options are passed through
Affected upstream fixtures:
- test/e2e/app-dir/scss/basic-module-include-paths
- test/e2e/app-dir/scss/basic-module-additional-data
- test/e2e/app-dir/scss/basic-module-prepend-data
Refs: cloudflare/vinext CI run 25951111875
Source: packages/next/src/build/webpack/config/blocks/css/index.ts
* fix(css): address sass passthrough review feedback
- Drop `export` on `VitePreprocessorOptions` so Knip stops flagging it
as unused (CI Check blocker).
- Use truthy-OR (`prependData || additionalData`) to match Next.js's
`sassPrependData || sassAdditionalData` exactly — empty-string
`prependData` now falls through to `additionalData` instead of
overriding it.
- Widen `additionalData` type to accept Vite's function form
(`(source, filename) => string | Promise<string>`) and drop the
inaccurate `as string` cast.
- Add tests covering the empty-`prependData` fallthrough and the
function-form `additionalData` passthrough.
James Anderson