- relay: SSE stream error exposure improvements (Claude/Gemini/OpenAI)
- test: add mock capture server for E2E relay capture testing

Co-Authored-By: Claude <noreply@anthropic.com>

- Gemini stream: 捕获 promptFeedback.blockReason 后停止流并返回 OpenAPI 错误
- OpenAI stream: 检测 SSE 帧中的 error 响应，在首帧写入前转换为 API 错误
- Responses stream: 处理 response.error / response.failed / error 类型帧
- 新增 ClearEventStreamHeadersIfNotWritten 工具方法，错误转换时清除流式头
- 新增 streamOpenAIErrorFromData，从 SSE data 帧提取 OpenAPI 错误结构
- 上游错误体会截断后存入 UpstreamBody 供排障

Co-Authored-By: Claude <noreply@anthropic.com>