refactor: Enhance docs, code, add tests in HarshadNumber

[Hardvan]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.77%. Comparing base (219cc33) to head (39a6969).

Files with missing lines	Patch %	Lines
...in/java/com/thealgorithms/maths/HarshadNumber.java	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6745      +/-   ##
============================================
- Coverage     77.78%   77.77%   -0.01%     
- Complexity     6302     6304       +2     
============================================
  Files           728      728              
  Lines         20986    20992       +6     
  Branches       4091     4092       +1     
============================================
+ Hits          16323    16326       +3     
- Misses         4021     4022       +1     
- Partials        642      644       +2     

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