Function classify 

pub fn classify(error: &Error) -> FailureClass

Classify an anyhow::Error into a FailureClass.

The error is expected to come from agent.run_streaming() after oxi-agent’s internal retries are exhausted (typically as AgentError::RetriesExhausted { last_error } or AgentError::Stream(...)). The classification is best-effort; when in doubt, returns FailureClass::Unknown.