feat: add Astraflow provider support

[ucloudnb666]

Summary

Adds support for Astraflow by UCloud as an OpenAI-compatible LLM backend for sketch.

Astraflow is an OpenAI-compatible AI model aggregation platform supporting 200+ models. Because its API is fully OpenAI-compatible, no new SDK or subpackage is needed — the integration simply detects the ASTRAFLOW_API_KEY (global) or ASTRAFLOW_CN_API_KEY (China) environment variables and transparently configures lambdaprompt to point at the correct Astraflow base URL.

How it works

At import time, pandas_extension.py now checks for Astraflow env vars and, if found, sets the OPENAI_API_KEY and OPENAI_API_BASE environment variables that lambdaprompt reads, so all existing prompt machinery works unchanged.

Usage

Global endpoint:

import os
os.environ['ASTRAFLOW_API_KEY'] = 'your-astraflow-api-key'
os.environ['SKETCH_USE_REMOTE_LAMBDAPROMPT'] = 'False'

import sketch
df.sketch.ask("Which columns contain PII?")

China endpoint:

import os
os.environ['ASTRAFLOW_CN_API_KEY'] = 'your-astraflow-cn-api-key'
os.environ['SKETCH_USE_REMOTE_LAMBDAPROMPT'] = 'False'

import sketch
df.sketch.ask("Which columns contain PII?")

Sign up / API keys:

• Global: https://astraflow.ucloud-global.com
• China: https://astraflow.ucloud.cn

Changes

• sketch/pandas_extension.py — auto-configure lambdaprompt when Astraflow env vars are present
• README.md — document Astraflow as a supported backend