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.

docs/client-integration.md

@@ -21,6 +21,7 @@ Metapi 暴露标准 OpenAI / Claude 兼容接口，下游客户端只需配置
 
 | 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） |