Add FriendlyNumber Implementation

[Rosander0]

• 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 algorithms include a corresponding test class that validates their functionality.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.88%. Comparing base (631eae3) to head (c7dbdce).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7474      +/-   ##
============================================
+ Coverage     79.86%   79.88%   +0.01%     
- Complexity     7333     7340       +7     
============================================
  Files           808      809       +1     
  Lines         23833    23847      +14     
  Branches       4690     4694       +4     
============================================
+ Hits          19035    19050      +15     
+ Misses         4036     4035       -1     
  Partials        762      762              

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[Copilot]

Pull request overview

Adds a new number-theory utility for detecting friendly numbers (numbers with the same abundancy index) along with a JUnit test suite to validate core behavior.

Changes:

• Introduces FriendlyNumber with areFriendly(a, b) based on divisor-sum (sigma) computation.
• Adds FriendlyNumberTest covering friendly, non-friendly, and invalid input cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/main/java/com/thealgorithms/maths/FriendlyNumber.java	Implements friendly-number check via sum-of-divisors and cross-multiplication comparison.
src/test/java/com/thealgorithms/maths/FriendlyNumberTest.java	Adds unit tests for expected friendly/non-friendly pairs and invalid inputs.

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