Fix CSV reader to handle 8-bit integers

[Alex-PLACET]

Checklist

• The title and commit message(s) are descriptive.
• Small commits made to fix your PR have been squashed to avoid history pollution.
• Tests have been added for new features or bug fixes.
• API of new functions and classes are documented.

Description

Address #2453

This PR fixes xcsv parsing for 8-bit integer types and adds focused test coverage around CSV load and dump behavior.

• fix lexical_cast for signed char and unsigned char so CSV values like 0 and 1 are parsed numerically instead of through character extraction
• fix string cell trimming so leading and trailing spaces are removed correctly

[JohanMabille]

Thanks!