fix: resolve binary addition and substring issues in Karatsuba algorithm

fix: correct binary addition and substring handling in the Karatsuba algorithm

Fixed the add_strings function to handle binary addition correctly.
Improved the safe_substr function to manage leading zeros and extract substrings properly.
Ensured correct bit shifting by using 1LL << for 64-bit integers.
Added comprehensive self-tests to verify functionality.

This fixes incorrect results from the Karatsuba algorithm for binary multiplication.

[ ] Added description of change
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[ ] Added tests and example, test must pass
[ ] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
[ ] Relevant documentation/comments is changed or added
[ ] PR title follows semantic commit guidelines
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Notes:

TheAlgorithms / C-Plus-Plus