[fix][Unplugin] Fix prod build CSS file name hashing

[nmn]

What changed / motivation ?

There is a bug with the unplugin package today where the StyleX styles don't affect the asset hash. This PR fixes that bug.

I tested the docs website and the basic Vite example and both are behaving as expected.

[vercel]

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Project	Deployment	Review	Updated (UTC)
stylex	Ready	Preview, Comment	Jan 10, 2026 3:29am

[github-actions]

workflow: benchmarks/perf

Comparison of performance test results, measured in operations per second. Larger is better.
yarn workspace v1.22.22
yarn run v1.22.22
$ node ./compare.js /tmp/tmp.JMylXQEPN9 /tmp/tmp.jdBTP2RwBP

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	554	558	1.01	+
· complex create	69	68	0.99	-
babel-plugin: stylex.createTheme
· basic themes	450	443	0.98	-
· complex themes	35	35	1.00
Done in 0.07s.
Done in 0.30s.

[github-actions]

workflow: benchmarks/size

Comparison of minified (terser) and compressed (brotli) size results, measured in bytes. Smaller is better.
yarn workspace v1.22.22
yarn run v1.22.22
$ node ./compare.js /tmp/tmp.3Ev5VY4wR9 /tmp/tmp.6YppukBDdw

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,311	1,311	1.00
· minified	4,150	4,150	1.00
@stylexjs/stylex/lib/cjs/inject.js
· compressed	1,793	1,793	1.00
· minified	4,915	4,915	1.00
benchmarks/size/.build/bundle.js
· compressed	496,650	496,650	1.00
· minified	4,847,840	4,847,840	1.00
benchmarks/size/.build/stylex.css
· compressed	99,853	99,853	1.00
· minified	747,541	747,541	1.00
Done in 0.09s.
Done in 0.35s.