perf: no need to init Rspack config before preview

[chenjiahan]

Summary

No need to init the Rspack config before rsbuild preview.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[netlify]

✅ Deploy Preview for rsbuild ready!

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🔨 Latest commit	933a7c0
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Lighthouse	1 paths audited Performance: 72 (no change from production) Accessibility: 97 (no change from production) Best Practices: 100 (no change from production) SEO: 92 (no change from production) PWA: 60 (no change from production) View the detailed breakdown and full score reports

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