Added HappyNumber algorithm in maths section

[ruturajjadhav07]

Description

This PR adds an implementation of the Happy Number algorithm in HappyNumber.java along with corresponding unit tests in HappyNumberTest.java.

• Implements cycle detection (Floyd’s Tortoise & Hare method) for efficiency.
• Includes detailed Javadoc and examples.
• Unit tests cover known happy numbers, unhappy numbers, and large input cases.

• 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: Happy Number
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

Notes

• Added both the algorithm and its unit tests.
• Used Wikipedia reference for algorithm definition.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.24%. Comparing base (5e9d9f7) to head (16204be).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6571      +/-   ##
============================================
+ Coverage     75.23%   75.24%   +0.01%     
- Complexity     5626     5631       +5     
============================================
  Files           691      692       +1     
  Lines         19448    19461      +13     
  Branches       3758     3761       +3     
============================================
+ Hits          14632    14644      +12     
- Misses         4248     4249       +1     
  Partials        568      568              

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