Function flatten_json_for_csv 

pub fn flatten_json_for_csv(value: &Value) -> (Vec<String>, Vec<Vec<String>>)

Flattens a JSON Value into a list of records for CSV export.

Returns a tuple of (headers, rows), where rows are vectors of strings corresponding to headers.

• If value is an Array, each element becomes a row.
• If value is an Object, it becomes a single row.
• Nested objects are flattened with dot notation.
• Arrays inside objects are serialized as JSON strings.