fix(proxy): fail over on empty upstream responses

[g1331]

Summary

修复上游返回空响应时被透传到下游的问题。成功但空 body 的非流式响应，以及没有产生有效内容就结束的 SSE 流，都会在响应提交前触发故障转移。

Related Issue

未关联具体 Issue。

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Tests (adding or updating tests)

Changes

• 新增 UpstreamEmptyResponseError，拦截成功状态下的空非流式响应。
• 空 SSE 流不再依赖 firstByteTimeout 配置才进行检测。
• 将空响应加入现有故障转移、失败记录和失败规则类型。
• 增加 proxy-client 与代理路由回归测试，并补充中英文显示文本。

Test Plan

• Local tests pass（相关单元测试：proxy-client 94/94，代理路由 88 通过）
• Type check passes（pnpm exec tsc --noEmit）
• Lint passes（相关文件 ESLint；提交钩子 ESLint）
• Manual testing completed

Checklist

• Code follows the project's coding standards
• Tests have been added where necessary
• Documentation has been updated（不适用）
• Changes do not introduce security vulnerabilities
• Commit messages follow conventions

Screenshots

不适用。

Additional Notes

截图中的 HTTP 499 仍表示下游客户端断开，不属于本次空响应错误。

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.13%. Comparing base (e391757) to head (33e804a).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #246      +/-   ##
==========================================
+ Coverage   72.09%   72.13%   +0.04%     
==========================================
  Files         243      243              
  Lines       14147    14152       +5     
  Branches     4674     4676       +2     
==========================================
+ Hits        10199    10209      +10     
  Misses       2598     2598              
+ Partials     1350     1345       -5     

Flag	Coverage Δ
verify	72.13% <93.33%> (+0.04%)	⬆️

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