fix: use correct proxy config in preview

[bluwy]

Description

Fixes #7414
Supersedes #7415

Use the preview proxy config in preview server

Additional context

I only noticed #7415 was created after I finished this 😬 But I think this method is a bit more robust. Thanks to @Jinjiang for the pointers in the issue, and @LIUYANG132315 for the initial work.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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