Function classify_completion_failure 

pub fn classify_completion_failure(body: Option<&str>) -> FailureClass

Classify a non-streaming completion body that failed acceptance: empty content → EmptyCompletion, truncated (finish_reason “length”) → TruncatedCompletion, invalid JSON → InvalidJson, wrong shape → InvalidChatCompletionShape, missing/null finish reason → MissingFinishReason, else Unknown.