big_integer/multiply: improve docs, validation, and clarity

[KushalMeghani1644]

This PR improves the multiply implementation in big_integer/multiply.rs:

1. Added clearer documentation with examples and panic conditions

2. Replaced char::is_numeric with is_ascii_digit to ensure strict base-10 validation

3. Improved panic messages for invalid inputs

4. Simplified multiplication loop for better readability

5. Renamed some test cases for clarity

Overall, this is a non-breaking change that improves clarity, correctness, and documentation.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.32%. Comparing base (4eec0b7) to head (57d4411).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #906   +/-   ##
=======================================
  Coverage   95.32%   95.32%           
=======================================
  Files         319      319           
  Lines       20807    20812    +5     
=======================================
+ Hits        19834    19839    +5     
  Misses        973      973           

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