Add configurable OpenAI API URL for OpenAI-compatible providers

[Alex-wuhu]

Summary

• make OPENAI_API_URL configurable via environment variable in Mobile-Agent-E/inference_agent_E.py
• document how to use OpenAI-compatible endpoints such as Novita in Mobile-Agent-E/README.md and README_zh.md

Example

export BACKBONE_TYPE="OpenAI"
export OPENAI_API_URL="https://api.novita.ai/openai/v1/chat/completions"
export OPENAI_API_KEY="your-novita-key"

Validation

• python3 -m py_compile Mobile-Agent-E/inference_agent_E.py