Function format_csv 

pub fn format_csv(rows: &[Map<String, Value>], bom: bool) -> String

Render rows as RFC 4180 CSV.

Header columns are the union of keys across all rows, preserved in insertion order from first occurrence. Rows missing a key emit an empty cell. Non-scalar values are serialized as canonical JSON inside the cell. Cells containing ,, ", \n, or \r are quote-wrapped with embedded " doubled. Line terminator is CRLF.