perf(strings): optimize LongestCommonSubstring DP memory to O(N) and …

[irenemrtinez]

Summary

• Memory Optimization: Refactored LongestCommonSubstring to use a 1D DP array (O(min(N, M)) dynamic memory) instead of a 2D matrix (O(N * M)).
• Performance Improvements: Prevents memory overload and timeouts when running string comparisons on large inputs ($4000+$ characters). With the old code, the new test I created failed
• Test Coverage: Added comprehensive unit tests in LongestCommonSubstringTest.java covering unicode characters, whitespace, repeated patterns, null/empty inputs, and boundary conditions.

Checklist

• 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 81.18%. Comparing base (87edc62) to head (63a4aba).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7613      +/-   ##
============================================
- Coverage     81.19%   81.18%   -0.02%     
+ Complexity     7900     7897       -3     
============================================
  Files           830      830              
  Lines         25012    25012              
  Branches       4882     4882              
============================================
- Hits          20308    20305       -3     
  Misses         3920     3920              
- Partials        784      787       +3     

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