feat(websearch): 新增Exa搜索提供商，支持 Tavily/Exa API Base URL 可配置，补充搜索工具相关文档#7359

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piexian wants to merge 6 commits intoAstrBotDevs:masterfrom

piexian:feat/exa-search-provider

Commits on Apr 6, 2026

fix(websearch): 修复 UUID 生成逻辑，确保唯一性；更新 API Base URL 错误提示信息；新增消息引用缓存机制
 piexian
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fix(websearch): 放宽 API Base URL 校验，增强 Tavily/Exa 请求报错提示
 piexian
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