Memory mapped datasource does not allow reading data beyond the mapped range

[vuule]

When creating a memory mapped datasource, we optionally pass a range within the file that we want mapped. Primary use for this is to avoid memory mapping the entire file when using a byte_range option in CSV/JSON.
Because we often need data beyond the exact byte_range, the mapped source add padding to the mapped range. However, we cannot guarantee that the reads will fall into this range.
Currently the source does not read beyond the mapped range and this can lead to incorrect output when the padding is not sufficient.
https://github.com/rapidsai/cudf/blob/branch-24.04/cpp/src/io/utilities/datasource.cpp#L163

Desired behavior:
Memory mapped datasource should read from the file when the mapping is not sufficient instead of clamping the returned data to the mapped range.

[vuule]

fixed by #16865