fix(gemini_client): 优化消息转换逻辑并改进错误提示

[xiaoxiao-cvs]

1. • main 分支 禁止修改，请确认本次提交的分支 不是 main 分支
2. • 我确认我阅读了贡献指南
3. • 本次更新类型为：BUG修复
   • 本次更新类型为：功能新增
4. • 本次更新是否经过测试
5. 请填写破坏性更新的具体内容（如有）: 无
6. 请简要说明本次更新的内容和目的：

修复 Gemini 客户端 _convert_messages 函数的问题：

• 将错误信息（"你tm..."）替换为正常信息（
• 为被跳过的Tool角色消息添加debug日志提示
• 过滤 parts 为空的Content对象，避免Gemini API报错
• 仅含System消息导致 contents 为空时，抛出清晰的ValueError而非让请求到达API后报错

其他信息

• 关联 Issue： [Bug]: Gemini 客户端处理 System 消息及空内容导致 API 报错 #1494
• 截图/GIF：

• 附加信息: 通过单元测试验证了修复，空 contents 校验、空 parts 过滤和正常对话回归

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Pull request overview

This PR fixes critical message conversion bugs in the Gemini client that caused API errors when handling system-only messages or messages with empty content. The changes improve error handling and add diagnostic logging to prevent Gemini API errors related to empty contents or parts fields.

Changes:

• Replaced unprofessional error message with clear, descriptive text
• Added debug logging for skipped Tool role messages
• Implemented filtering to remove Content objects with empty parts arrays
• Added validation to ensure contents list is never empty before API calls
• Simplified return statement logic for better readability

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