fix: conditionally inject @import or @reference based on file type to fix css modules dedup without breaking global css

[colinaaa]

Summary

• Use @reference instead of @import for .module.css (and other CSS modules) to resolve CSS modules duplication issues.
• Retain @import for regular CSS files to ensure the theme :root variables are correctly emitted, which fixes the regression in CSS-only pipelines and custom CSS files that use theme variables (like test/functions).

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