updateItem performs a full replacement of a selected form item. It preserves
the existing item ID and position, and corresponding question IDs where possible.
Example
Inputs
form, itemReference (immutable item ID or one-based position), and a complete
replacement item are required. Item definitions match Add item.
requiredRevisionId is optional.
account optionally selects the Google account binding.
Output
Returns the complete updatedForm. Because the update mask is the whole item,
omitted replacement fields are removed rather than preserved.