fix(packages/sui-bundler): keep plainCssPackages rules out of the link sass loader

[tomasmax]

Problem

plainCssPackages (added in #1988, released in v9.79.0) has no effect when the bundler runs with link flags (-l / -L).

loaders/linkLoaderConfigBuilder.js rewrote every rule whose test matched .css, swapping its last loader for @s-ui/sass-loader with the link importer. That also hit the plainCssPackages passthrough rule ([style-loader, css-loader]), turning it into [style-loader, @s-ui/sass-loader]. Plain CSS then went through Sass and the build died:

ERROR in node_modules/@adv-mt/ui/dist/basic/button/styles.css
Module build failed (from node_modules/@s-ui/sass-loader/src/index.js):
Error: Expected digit.
2 │ @layer properties{@supports (((-webkit-hyphens:none)) and ...

Reproduced in frontend-mt--web-app, whose dev_coches target passes -l ../packages/domain … -L ../packages/ui and declares plainCssPackages: ["@adv-mt/ui", "@adv-mt/theme"] (Tailwind v4 output — not valid SCSS).

Fix

Only replace the sass importer on rules that already pipe through @s-ui/sass-loader. Any other rule is returned untouched, so the plainCssPackages passthrough survives link mode. Also bails out when use isn't an array.

Verification

Ran the builder over a config mirroring webpack.config.dev.js (sass rule + plainCssPackages passthrough + a JS rule) with packagesToLink set:

• sass rule → last loader still swapped for the importer-aware @s-ui/sass-loader ✅
• plainCssPackages rule → [style-loader, css-loader], unchanged ✅
• JS rule → unchanged ✅

Before the fix the second rule became [style-loader, @s-ui/sass-loader]. sui-lint js reports 0 errors on the touched file (only pre-existing sui/commonjs warnings).

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