feat: enhance upstream endpoint handling with new passthrough header extraction and response management

- Introduced new types for endpoint preferences to support 'responses'.
- Added functions to extract safe passthrough headers, Claude-specific headers, and responses-specific headers.
- Implemented logic to ensure proper handling of stream accept headers.
- Updated endpoint resolution logic to prioritize 'responses' in certain contexts.
- Enhanced request building for 'messages' and 'responses' endpoints with improved body sanitization and header management.
- Added comprehensive documentation for new features and usage instructions.

docs/client-integration.md

@@ -64,9 +64,16 @@ Metapi 暴露标准 OpenAI / Claude 兼容接口，下游客户端只需配置
 ```bash
 export ANTHROPIC_BASE_URL=https://your-domain.com
 export ANTHROPIC_API_KEY=your-proxy-sk-token
+export CLAUDE_CODE_ATTRIBUTION_HEADER=0
+export CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC=1
+# 可选：进一步关闭非必要模型调用
+export DISABLE_NON_ESSENTIAL_MODEL_CALLS=1
 ```
 
-或在配置文件中设置相应的环境变量。
+或在配置文件中设置相应的环境变量（例如 `~/.claude/settings.json`）。
+
+> 说明：上述变量由 Claude Code 客户端读取，属于客户端行为开关；
+> `metapi` 服务端无法替代客户端“设置这些变量”，只能处理已经发来的请求。
 
 ### Roo Code / Kilo Code