feat: add Astraflow provider support

[ucloudnb666]

Summary

Adds Astraflow (by UCloud / 优刻得) as a native provider in MingLi-Bench.

Astraflow is an OpenAI-compatible AI model aggregation platform supporting 200+ models, available via both a global endpoint and a China endpoint.

• Global endpoint: https://api-us-ca.umodelverse.ai/v1 — env var ASTRAFLOW_API_KEY
• China endpoint: https://api.modelverse.cn/v1 — env var ASTRAFLOW_CN_API_KEY
• Website: https://astraflow.ucloud-global.com (global) / https://astraflow.ucloud.cn (China)

Changes

File	Change
mingli_bench/models/astraflow_client.py	New OpenAI-compatible client (mirrors DeepSeek pattern); includes both AstraflowClient (global) and AstraflowCNClient (China)
mingli_bench/models/factory.py	Register both providers, SDK install hint, astraflow-* model prefix detection, example model list
mingli_bench/utils/config.py	Add astraflow + astraflow_cn config blocks reading from env vars
.env.example	Document ASTRAFLOW_API_KEY / ASTRAFLOW_CN_API_KEY env vars

Usage

from mingli_bench.models.factory import ModelFactory

# Global endpoint (ASTRAFLOW_API_KEY)
client = ModelFactory.create("astraflow-deepseek-ai/DeepSeek-R1", provider="astraflow")

# China endpoint (ASTRAFLOW_CN_API_KEY)
client = ModelFactory.create("astraflow-deepseek-ai/DeepSeek-R1", provider="astraflow_cn")

Because the API is fully OpenAI-compatible, no new SDK is required — the existing openai package is reused.