fix: resolve binary addition and substring issues in Karatsuba algorithm

[ShanawazAlam007]

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.

Description of Change

Checklist

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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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