Update karatsuba_algorithm_for_fast_multiplication.cpp

[Lakshya747]

Description of Change

Key Improvements:
1.Simplified add_strings: The addition function now directly calculates the carry and adds bits from right to left, handling overflow more clearly.
2.Handling Leading Zeros: The algorithm checks for special cases directly without needing to modify the strings multiple times.
3.Efficiency: The code uses std::stoll for conversions instead of character arithmetic when multiplying by constants like 1 and 0.
4.Code Clarity: The comments and structure have been cleaned up for better readability.

Checklist

• Added description of change
• Added file name matches File name guidelines
• 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
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes:

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