fix(mergeConfig): avoiding adding undefined properties

[chenjiahan]

Summary

Avoiding adding undefined dev or output properties when calling the mergeRsbuildConfig method.

Checklist

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

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[Copilot]

Pull Request Overview

This PR fixes a bug in the normalizeConfigStructure function where config properties could be lost during normalization. The refactor ensures all original config properties are preserved by creating a shallow copy upfront and only overwriting normalized properties when they exist.

• Replaced destructuring with rest operator (...rest) with a full shallow copy of the config
• Changed from conditionally normalizing properties inline to explicitly assigning them back to the config copy
• Fixed the return statement to return the complete normalized config instead of reconstructing it

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