feat: Add Astraflow LLM provider support (global & China endpoints)

[ucloudnb666]

Summary

This PR adds Astraflow (by UCloud / 优刻得) as a supported LLM provider in MetaGPT.

Astraflow is an OpenAI-compatible AI model aggregation platform supporting 200+ models. It exposes two regional endpoints:

Variant	Env Var	Base URL
Global	ASTRAFLOW_API_KEY	https://api-us-ca.umodelverse.ai/v1
China	ASTRAFLOW_CN_API_KEY	https://api.modelverse.cn/v1

Sign up at: https://astraflow.ucloud.cn/

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Changes

1. metagpt/configs/llm_config.py

• Added ASTRAFLOW = "astraflow" and ASTRAFLOW_CN = "astraflow_cn" to the LLMType enum.

2. metagpt/provider/astraflow_api.py (new file)

• Created AstraflowLLM — a thin subclass of OpenAILLM registered for both LLMType.ASTRAFLOW and LLMType.ASTRAFLOW_CN.
• Automatically sets the correct base_url based on the api_type when none is explicitly configured.

3. metagpt/provider/__init__.py

• Imported and exported AstraflowLLM.

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Usage

Global endpoint (config2.yaml):

llm:
  api_type: "astraflow"
  api_key: "YOUR_ASTRAFLOW_API_KEY"   # env: ASTRAFLOW_API_KEY
  model: "YOUR_MODEL_NAME"            # e.g. gpt-4o, claude-3-5-sonnet, etc.

China endpoint (config2.yaml):

llm:
  api_type: "astraflow_cn"
  api_key: "YOUR_ASTRAFLOW_CN_API_KEY"  # env: ASTRAFLOW_CN_API_KEY
  model: "YOUR_MODEL_NAME"

Both variants inherit all capabilities of OpenAILLM (streaming, tool calls, image generation, cost tracking, etc.) since Astraflow is fully OpenAI-compatible.