OpenAI 服务类自定义 503 status code (no body)

### Bug Description

qwen3.5-9b 走自定义模型，其余默认走qwen

请求日志，已路由到自定义的 lmstudio-ai-route：

{"ai_log":"-","authority":"aigw-local.hiclaw.io","bytes_received":"100","bytes_sent":"0","downstream_local_address":"172.18.0.3:8080","downstream_remote_address":"172.18.0.1:59780","duration":"3","istio_policy_status":"-","method":"POST","path":"/v1/chat/completions","protocol":"HTTP/1.1","request_id":"39263ce2-d8e8-4826-acca-b8e9d4c60dfd","requested_server_name":"-","response_code":"503","response_flags":"NC","route_name":"ai-route-lmstudio-ai-route.internal","start_time":"2026-04-02T07:27:11.570Z","trace_id":"-","upstream_cluster":"-","upstream_host":"-","upstream_local_address":"-","upstream_service_time":"-","upstream_transport_failure_reason":"-","user_agent":"curl/8.7.1","x_forwarded_for":"172.18.0.1","response_code_details":"cluster_not_found"}

帮助文档也查看过：[如果 Manager 没有响应，或者返回了 404、503 等状态码](https://github.com/agentscope-ai/HiClaw/blob/main/docs/zh-cn/faq.md#%E5%9C%A8%E6%88%BF%E9%97%B4%E9%87%8C%E5%92%8C-manager-%E8%81%8A%E5%A4%A9%E6%B2%A1%E6%9C%89%E5%93%8D%E5%BA%94%E6%88%96%E8%BF%94%E5%9B%9E%E9%94%99%E8%AF%AF%E7%8A%B6%E6%80%81%E7%A0%81)

文档结论”要判断是后端服务出错还是 Higress 自身配置问题，查看日志中的 upstream_host 字段：如果该字段有值，说明请求已到达后端，异常状态码是由上游服务返回的；如果为空，说明 Higress 本身无法路由该请求“

实际测试 BaseURL 在容器和宿主机都可以正常访问：
```
curl http://host.docker.internal:1234/v1/chat/completions \
-H "Content-Type: application/json" \
-d '{
"model": "qwen3.5-9b@q6_k_xl",
"messages": [
{
"role": "user",
"content": "你好"
}
]
}'> > > > > > > > > >
{
"id": "chatcmpl-wfqgiu1uykq5q356gek96k",
"object": "chat.completion",
"created": 1775113589,
"model": "qwen3.5-9b@q6_k_xl",
"choices": [
{
"index": 0,
"message": {
"role": "assistant",
"content": "你好！有什么我可以帮助你的吗？无论是回答问题、创作内容，还是分析数据，随时告诉我你的需求吧！ 😊",
"reasoning_content": "",
"tool_calls": []
},
"logprobs": null,
"finish_reason": "stop"
}
],
"usage": {
"prompt_tokens": 13,
"completion_tokens": 27,
"total_tokens": 40,
"completion_tokens_details": {
"reasoning_tokens": 0
}
},
"stats": {},
```

### Steps to Reproduce

无

### AI Analysis

_No response_

### Relevant Logs

```shell

```

### Component

Manager Agent

### Version / Commit

_No response_