Fix binary addition logic in add_strings function

[chitvs]

Description of Change

This pull request addresses a bug in the add_strings function located in divide_and_conquer/karatsuba_algorithm_for_fast_multiplication.cpp. The original implementation incorrectly used bitwise XOR and character casting for binary addition, leading to incorrect results during binary multiplication. The updated code uses proper integer arithmetic to handle binary addition correctly, ensuring accurate summation of binary strings.

Fixes #2848 (Incorrect Binary Addition Logic in add_strings).

Checklist

• [x] Added description of change
• [x] Added file name matches File name guidelines
• [x] Added tests and example, test must pass
• [x] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• [x] Relevant documentation/comments is changed or added
• [x] PR title follows semantic commit guidelines
• [x] Search previous suggestions before making a new one, as yours may be a duplicate.
• [x] I acknowledge that all my contributions will be made under the project's license.

Notes: This fix corrects the binary addition logic, ensuring accurate results for binary multiplication.

[chitvs]

Hi @Panquesito7 and @realstealthninja, is there anything else needed to finalize this PR? Thanks!

Best, chitvs