Function convert_and_bound_idx_ca 

pub fn convert_and_bound_idx_ca<T>(
    ca: &ChunkedArray<T>,
    target_len: usize,
    null_on_oob: bool,
) -> PolarsResult<IdxCa>
where
    T: PolarsIntegerType,
    T::Native: ToPrimitive,

UNSIGNED conversion:

• 0 <= v < target_len → Some(v)
• v >= target_len → None

SIGNED conversion with Python-style negative semantics:

• v < -target_len → None
• -target_len <= v < 0 → Some(target_len + v)
• 0 <= v < target_len → Some(v)
• v >= target_len → None