Function insert_num_fmt 

pub fn insert_num_fmt(xml: &str, cell_index: usize, format: &str) -> String

Inserts <w:numFmt w:val="FORMAT"/> into the paragraph run-properties of the cell at cell_index.

The number format is placed inside <w:rPr> that appears as a direct child of <w:pPr> (the paragraph-level default run properties), which is the canonical location for cell-level number formatting in OOXML.