updateRows writes only the supplied cells and preserves other columns.
Example
Inputs
spreadsheet, source, and non-empty updates are required. Each update has a
one-based rowNumber and non-empty values record whose keys exactly match
headers. null clears a cell. inputMode defaults to userEntered; account
selects the Google account.
Output
Returns{ affectedCells, rowNumbers }. affectedCells counts supplied cells,
not the full row width. Duplicate row numbers are accepted and remain in output;
later writes to the same cell win in request order.