新增插件测试指南文档（中英双语）并更新侧边栏

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• Upstream PR: test: enhance test framework with comprehensive fixtures and mocks AstrBot#5354
• Trigger: PR merged

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AI change summary:

• 新增插件测试指南文档 zh/dev/star/guides/testing.md 与 en/dev/star/guides/testing.md，涵盖测试框架结构、Fixtures、平台适配器 Mock 及最佳实践。
• 更新 .vitepress/config.mjs，在“插件开发”侧边栏中添加“插件测试”导航链接。
• i18n：已同步中英文版本。

Please review manually before merging.

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• This output is generated by AstrBot-Doc-Agent for review only.
• It does not represent the final documentation form.

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