Function extract_assistant_content 

pub fn extract_assistant_content(root: &Value) -> Option<String>

Extract the assistant’s text content from an LLM API response.

Handles various response formats (tried in order):

• OpenAI: choices[0].message.content string or array of content blocks
• Anthropic Messages API: root-level content array with type: "text" blocks
• output_text field (some providers)