Function save_query_result 

pub fn save_query_result(
    question: &str,
    answer: &str,
    memory_dir: &Path,
    query_type: &str,
    source_nodes: Option<&[String]>,
) -> Result<PathBuf, IngestError>

Save a query result (question + answer) to the memory directory.

Used by the save-result CLI command to persist LLM query results for future reference.