perf: skip assets traversal if source map is disabled

[chenjiahan]

Summary

If source map generation is disabled, it may take a few milliseconds to traverse all assets. This PR resolved the issue.

Checklist

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

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[Copilot]

Pull Request Overview

This PR optimizes the source map plugin by skipping asset traversal when source map generation is disabled, reducing unnecessary processing time by a few milliseconds.

Key Changes:

• Extracted CSS source map check into a reusable enableCssSourceMap helper function
• Added early return in the processAssets hook when both JS and CSS source maps are disabled

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