修复 FetchTask 不遵循代理设置的问题

[Glavo]

No description provided.

[Copilot]

Pull Request Overview

This PR fixes an issue where FetchTask doesn't respect proxy settings by implementing lazy initialization of the HTTP client. The key change moves the HTTP_CLIENT initialization from static class loading time to on-demand initialization via a holder pattern.

• Moved HTTP_CLIENT from static field to lazy-initialized holder class
• Changed HTTP_CLIENT reference to use Holder.HTTP_CLIENT pattern
• Added documentation explaining the initialization timing requirement

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