feat: add Astraflow provider support

[ucloudnb666]

Summary

Adds Astraflow by UCloud as a supported LLM provider.

Astraflow is an OpenAI-compatible AI model aggregation platform supporting 200+ models. Because it is OpenAI-compatible, no new SDK or dependency is needed — users simply pass base_url and api_key to the existing ChatOpenAI constructor:

from langchain_openai import ChatOpenAI

# Global endpoint
llm = ChatOpenAI(
    model_name="gpt-4o",           # or any of the 200+ supported models
    openai_api_key=os.getenv("ASTRAFLOW_API_KEY"),
    openai_api_base="https://api-us-ca.umodelverse.ai/v1",
    temperature=0.1,
)

# China endpoint
llm = ChatOpenAI(
    model_name="gpt-4o",
    openai_api_key=os.getenv("ASTRAFLOW_CN_API_KEY"),
    openai_api_base="https://api.modelverse.cn/v1",
    temperature=0.1,
)

Changes

• .env.example — added ASTRAFLOW_API_KEY (global) and ASTRAFLOW_CN_API_KEY (China endpoint) with sign-up URLs
• README.md — added Astraflow keys to the "Set Up Environment Variables" section so users know to populate them

Provider info

	Global	China
Website	https://astraflow.ucloud-global.com	https://astraflow.ucloud.cn
API key	https://astraflow.ucloud-global.com	https://astraflow.ucloud.cn
Base URL	https://api-us-ca.umodelverse.ai/v1	https://api.modelverse.cn/v1
Env var	ASTRAFLOW_API_KEY	ASTRAFLOW_CN_API_KEY