perf: lazy load open module to improve startup time

[chenjiahan]

Summary

The open package is only used when the dev --open flag is passed. Lazy loading this module can improve startup time.

Checklist

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

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[Copilot]

Pull Request Overview

This PR optimizes startup performance by lazy loading the open module only when the dev --open flag is used, rather than importing it at module initialization time.

Key Changes:

• Moved the open module import from top-level to within the openBrowser function using dynamic import

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