feat: Add support for OpenAI Responses endpoint and enhance proxy retry logic

- Updated proxyRetryPolicy to retry on all 4xx and 5xx status codes.
- Introduced 'responses' target format in TestChatRequestPayload.
- Implemented extractResponsesContent function to handle responses format in ModelTester.
- Enhanced ModelTester to support responses target format and display appropriate labels.
- Updated ProxyLogs to extract and display upstream paths from error messages.
- Added tests to ensure responses target format is preserved during payload sync.
- Created responses proxy route to handle requests to the new OpenAI Responses endpoint.
- Updated upstreamEndpoint logic to include responses endpoint handling.
- Added modelService tests to validate token route rebuilding logic.

README.md

@@ -100,7 +100,7 @@ Metapi 作为这些中转站之上的聚合层，把多个站点统一到 **一
 ### 🌐 统一代理网关
 
 - 兼容 **OpenAI** 与 **Claude** 下游格式，对接所有主流客户端
-- 支持 Chat Completions / Messages / Completions / Embeddings / Images / Models 全接口
+- 支持 Responses / Chat Completions / Messages / Completions（Legacy）/ Embeddings / Images / Models 全接口
 - 完整的 SSE 流式传输支持，自动格式转换（OpenAI ⇄ Claude）
 
 ### 🧠 智能路由引擎
@@ -341,6 +341,7 @@ Metapi 对下游暴露标准 OpenAI / Claude 兼容接口：
 
 | 接口                       | 方法 | 说明                         |
 | -------------------------- | ---- | ---------------------------- |
+| `/v1/responses`          | POST | OpenAI Responses             |
 | `/v1/chat/completions`   | POST | OpenAI Chat Completions      |
 | `/v1/messages`           | POST | Claude Messages              |
 | `/v1/completions`        | POST | OpenAI Completions（Legacy） |