Optimized the code of armstrong_number.cpp

[TechWhiz42]

Description of Change

• Fixed the edge case where 0 was treated as having 0 digits by correctly considering it a one-digit number.
• Replaced floating-point exponentiation (std::pow) with an integer exponentiation helper function to avoid unnecessary floating-point operations.
• Added additional self-test cases to improve test coverage for valid and invalid Armstrong numbers.
• Removed the unused <cmath> header after eliminating std::pow().

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
• 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: Improves correctness for the edge case of 0, replaces floating-point exponentiation with integer arithmetic, and expands the self-test suite.

[TechWhiz42]

Hey people,
This is my very first contribution. Even if my contribution is not merged, please tell me how can I improve. That will be more than enough.
Thank you.