The function is designed to handle various input formats encountered during
spreadsheet data processing. In addition to single column labels, it supports
range notation using the colon operator (e.g., "A:C"). When a range is
detected, the function internally expands the notation into a complete
sequence of integers (e.g., 1, 2, 3). This behavior is particularly useful
when passing column selections to functions like wb_to_df() or wb_read().
Input validation ensures that only atomic vectors are processed. If the input
is already numeric or a factor, the function ensures the values fall within
the valid spreadsheet column range before coercion to integers. Note that
the presence of NA values in the input will trigger an error to maintain
data integrity during index calculation.