Add integer to roman v2

[NA-V10]

Checklist

[x] I have read the contributing guidelines [x] I have added doctests [x] My code follows the project style [x] This algorithm is not a duplicate [x] All doctests pass locally

Summary

This PR adds the integer_to_roman algorithm to the strings directory. It converts an integer to a Roman numeral and includes doctests and input validation.

Description

Reversible integer-to-Roman mapping logic Valid for numbers 1–3999 Clear, descriptive parameter names as required by algorithms-keeper Pure Python implementation with tests

Thank you!