refactor: Enhance docs, code, add tests in KaprekarNumbers

[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 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.22%. Comparing base (ff9fd2e) to head (3f9ff01).

Files with missing lines	Patch %	Lines
.../java/com/thealgorithms/maths/KaprekarNumbers.java	95.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## master #6747 +/- ## ============================================ - Coverage 77.22% 77.22% -0.01%  - Complexity 6142 6144 +2  ============================================ Files 720 720 Lines 20527 20533 +6 Branches 3973 3974 +1 ============================================ + Hits 15853 15857 +4  Misses 4068 4068 - Partials 606 608 +2 

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[alxkm]

Looks good. Thank you for the contribution.