Function convert_to_unsigned_index 

pub fn convert_to_unsigned_index(
    s: &Series,
    target_len: usize,
    null_on_oob: bool,
) -> PolarsResult<IdxCa>

Convert arbitrary integer Series into IdxCa, using target_len as logical length.

• All OOB indices are mapped to null in convert_*.
• We track null counts before and after:
  • if null_on_oob == true, extra nulls are expected and we just return.
  • if null_on_oob == false and new nulls appear, we raise OutOfBounds.