fix(sui-bundler): don't force sass-loader on plainCssPackages when linking

[tomasmax]

Summary

• linkLoaderConfigBuilder (used by sui-bundler dev -l/-L and sui-bundler build -l/-L for local package linking) rewrites the last loader of every module rule whose test matches .css, injecting @s-ui/sass-loader with the link importer.
• This unconditionally includes the plainCssPackages passthrough rule added in feat(sui-bundler): add plainCssPackages config to bypass sass-loader #1988, which exists specifically to make webpack treat certain packages' .css as plain CSS and skip sass-loader entirely.
• Net effect: any consumer linking local packages (a standard monorepo dev workflow) while also using plainCssPackages for a dependency shipping modern CSS (e.g. Tailwind v4's @layer/@supports syntax) gets that CSS silently routed through sass-loader anyway, which can't parse it → Module parse failed at dev-server startup.
• Fix: only rewrite rules whose use chain already includes @s-ui/sass-loader, compared by require.resolved path (not a package-name substring — needed so this still works when @s-ui/sass-loader itself is linked, where its resolved path is packages/sui-sass-loader/src/index.js and doesn't contain the package name).

Test plan

• Added packages/sui-bundler/test/server/linkLoaderConfigBuilderSpec.js: asserts the sass-loader rule still gets the link importer injected, the plainCssPackages passthrough rule is left untouched, and the no-op case (nothing to link) returns config unchanged.
• npx @s-ui/test server -P 'packages/sui-bundler/test/server/*Spec.js' — 15 passing (includes pre-existing linkLoaderSpec, plainCssSplitChunksSpec, integration specs).
• sui-lint js --fix — 0 errors (pre-existing sui/commonjs warnings elsewhere in the package, not introduced by this change).
• Full pre-push suite (npm run test && npm run types:check) passed: 595 client tests, 18 server tests, typecheck clean.
• Reproduced against a real consumer (frontend-mt--web-app, running dev:coches with -l ../packages/domain -l ../packages/literals -l ../packages/segment-wrapper -l ../packages/theme -L ../packages/ui and plainCssPackages: ["@adv-mt/ui", "@adv-mt/theme"]): before the fix, dev server failed to compile with Module parse failed: Unexpected token on @adv-mt/theme/dist/tokens.css and @adv-mt/ui/dist/basic/button/styles.css; after patching locally, dev server compiled and served correctly.

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