Function extract_human_error_message 

pub fn extract_human_error_message(body: &str) -> String

Extract the most human-readable error message from a provider’s JSON error body.

Handles all known provider response schemas:

• OpenAI/DeepSeek/ZAI/Anthropic: {"error": {"message": "..."}}
• HuggingFace: {"error": "..."}
• Gemini: {"error": {"status": "..."}}
• FastAPI / OpenAI alternate: {"detail": "..."}
• Generic: {"message": "..."}

Falls back to the raw body if no known field is found.