Add Longest Repeated Substring algorithm (#7286)

* Add Longest Repeated Substring algorithm

Implement LongestRepeatedSubstring in the strings package using the
existing SuffixArray and Kasai's algorithm for LCP array construction.
Includes parameterized unit tests covering typical and edge cases.

* style: reformat test file per clang-format and add coverage test

---------

Co-authored-by: Çağlar Eker <eker.caglar@hepsiburada.com>