Function read_len_prefixed_bytes_at 

pub fn read_len_prefixed_bytes_at(
    data: &Bytes,
    offset: &mut usize,
) -> Result<Bytes, ArrowError>

Read a length-prefixed byte slice at offset in data, advancing offset.

Reads an 8-byte little-endian length, then slices exactly that many bytes from data. The returned Bytes is zero-copy (shares data’s allocation).