feat: add Astraflow (ModelVerse) provider support

[ucloudnb666]

Summary

This PR adds support for Astraflow (UCloud ModelVerse) as a model provider in Strands Agents.

Astraflow is UCloud's AI model aggregation platform that provides an OpenAI-compatible API supporting models such as DeepSeek, Claude, GPT-4, and more.

Two Regional Endpoints (registered as independent providers)

Provider class	Region	Base URL	API Key env var
AstraflowModel	Global (US/CA)	https://api-us-ca.umodelverse.ai/v1	ASTRAFLOW_API_KEY
AstraflowCNModel	China	https://api.modelverse.cn/v1	ASTRAFLOW_CN_API_KEY

Changes

• src/strands/models/astraflow.py — New provider module with AstraflowModel (global) and AstraflowCNModel (China) classes, both built on top of the existing OpenAIModel with pre-configured base_url and api_key defaults.
• src/strands/models/__init__.py — Lazy-load entries for AstraflowModel and AstraflowCNModel.
• pyproject.toml — New astraflow optional dependency group (reuses the openai package).

Usage

# Global node (US/CA)
from strands import Agent
from strands.models.astraflow import AstraflowModel

model = AstraflowModel(model_id="claude-3-5-haiku-20241022")
agent = Agent(model=model)
agent("Hello!")

# China node
from strands.models.astraflow import AstraflowCNModel

model_cn = AstraflowCNModel(model_id="deepseek-ai/DeepSeek-V3")
agent_cn = Agent(model=model_cn)
agent_cn("你好！")

Install with:

pip install "strands-agents[astraflow]"

Set your API key:

export ASTRAFLOW_API_KEY="your-global-api-key"
export ASTRAFLOW_CN_API_KEY="your-china-api-key"

Design Notes

• AstraflowModel and AstraflowCNModel extend OpenAIModel directly, inheriting all OpenAI-compatible streaming, tool-use, and structured-output logic.
• The base_url and api_key are pre-configured via client_args defaults, so users only need to set the environment variable — no manual URL configuration required.
• Follows the same pattern as other OpenAI-compatible providers in this repo (e.g., SageMakerAIModel).