fix(proxy): 区分上游空内容流与首字超时

[g1331]

Summary

修复 waitForFirstStreamContent 错误消息把"配置阈值"误当成"实际等待时长"的语义 bug。原先 streamDoneBeforeContentPromise 分支与真超时分支共用 FirstByteTimeoutError(timeoutMs)，导致上游 SSE 流在 2–5 秒内正常关闭却没产生 content-bearing chunk 时，UI 与日志固定显示 Upstream first byte timed out after 30s —— 实际只等了 3 秒。

• 新增 UpstreamNoContentStreamError，构造函数同时接收 elapsedMs（实际耗时）与 firstByteTimeoutMs（配置阈值），文案如实呈现 Upstream closed SSE stream after 3.58s without producing any content-bearing chunk (first-byte timeout config: 30s)。
• FailoverErrorType 增加 upstream_no_content_stream；getErrorType 与 isFailoverableError 单独识别该类，故障转移行为与之前一致。
• 顺手补齐 retryErrorType 字典里此前缺失的 first_byte_timeout / stream_idle_timeout / stream_error 三项中英文，避免运行时回落到原始枚举名。

Investigation evidence

生产环境 rc-cx-pro 上游六条 503 的实测耗时 vs 错误文案：

reqId	上游响应	耗时	错误文案
b19e6722	200 / text/event-stream	2.23 s	timed out after 30s
44eac652	200 / text/event-stream	3.63 s	timed out after 30s
6bff51e5	200 / text/event-stream	4.55 s	timed out after 30s
acfcc4c9	200 / text/event-stream	3.58 s	timed out after 30s
0b0f13f5	200 / text/event-stream	2.58 s	timed out after 30s
2ef5ecd3	200 / text/event-stream	2.68 s	timed out after 30s

firstByteTimeout 配置为默认 30 秒，但每条都在 2–5 秒结束。setTimeout 计时器没有触发，命中的是 streamDoneBeforeContentPromise 分支——上游正常完成了 SSE 流但只发送 metadata 事件。

Behavior changes

• 新场景的 error_type 由 first_byte_timeout 变更为 upstream_no_content_stream。之前依赖匹配 first_byte_timeout 的失败规则需要追加 upstream_no_content_stream 才能继续覆盖该场景。
• 历史 request_logs 行保留原 first_byte_timeout 值，不做迁移。
• 不引入 recordFailure 调用变更（电路熔断行为保持原状）；如需让该错误也计入熔断失败计数，应作为独立 PR 评估。

Test plan

• pnpm exec tsc --noEmit
• pnpm lint
• pnpm format:check
• pnpm test:run（147 文件 / 2487 用例全绿，其中 tests/unit/services/proxy-client.test.ts 的 path B 用例已改为期望新错误类）

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.15%. Comparing base (d565097) to head (a4b3d72).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
+ Coverage   74.14%   74.15%   +0.01%     
==========================================
  Files         145      145              
  Lines       11043    11048       +5     
  Branches     3832     3832              
==========================================
+ Hits         8188     8193       +5     
  Misses       1657     1657              
  Partials     1198     1198              

Flag	Coverage Δ
verify	74.15% <83.33%> (+0.01%)	⬆️

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