Add ReturnOnInvestment to maths

[InukaWijerathna]

Description

Adds ReturnOnInvestment to the maths package.

Return on Investment (ROI) is a fundamental financial metric that quantifies how profitable an investment is relative to its cost:

ROI = (Gain from Investment - Cost of Investment) / Cost of Investment × 100

What's included

• ReturnOnInvestment.java — clean implementation with Javadoc and IllegalArgumentException for invalid cost
• ReturnOnInvestmentTest.java — 6 JUnit 5 tests covering positive, zero, and negative ROI, plus error paths

Checklist

• Follows the code style of the project (final class, private constructor, static method)
• JUnit 5 tests for all branches including exception cases
• Javadoc with formula and reference link

Reference: https://www.investopedia.com/terms/r/returnoninvestment.asp

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.73%. Comparing base (cecfad9) to head (ff3feff).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #7446   +/-   ##
=========================================
  Coverage     79.72%   79.73%           
- Complexity     7298     7300    +2     
=========================================
  Files           803      804    +1     
  Lines         23762    23765    +3     
  Branches       4674     4674           
=========================================
+ Hits          18945    18948    +3     
  Misses         4059     4059           
  Partials        758      758           

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