fix(css): resolve tsconfig paths in CSS and Sass @import

[shulaoda]

Description

close #22766

Since the oxc-resolver upgrade in Vite 8.1, resolve.tsconfigPaths stopped resolving paths aliases inside CSS @import and Sass @use, such as @import "@/styles/main.css".

oxc-resolver only applies a tsconfig's paths to a file that tsconfig owns through its files or include. For CSS @import, Vite passed a synthetic <dir>/* importer that has no file extension, so no tsconfig ever owns it and the paths aliases are skipped.

The fix passes the real importing file as the importer: CSS (postcss-import) uses the @import AtRule's source file, and Sass already uses context.containingUrl. Because tsconfig matches by extension, CSS/Sass files must be declared explicitly to receive paths, for example include: ["src", "src/**/*.css", "src/**/*.scss"]. This is documented under resolve.tsconfigPaths.

Less is not supported: its plugin API only exposes the importer's directory, not the file, so Vite cannot tell the resolver which tsconfig owns it. Use a relative path or resolve.alias for @import in Less. This is documented and locked in by a negative test.

playground/resolve-tsconfig-paths gains CSS @import and Sass @use cases that resolve via paths, plus a Less case asserting the alias stays unresolved.

Additional

I looked into supporting Less by overriding getPath in its file-manager plugin so the importer's file (not just its directory) is exposed to Vite, but that repurposes a method Less relies on internally for directory derivation and is too hacky, so Less is left unsupported for now.