Add LongestCommonSubstring 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 79.83%. Comparing base (6fbbc94) to head (d2b2a0f).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7464      +/-   ##
============================================
+ Coverage     79.80%   79.83%   +0.03%     
- Complexity     7302     7314      +12     
============================================
  Files           803      804       +1     
  Lines         23751    23767      +16     
  Branches       4671     4676       +5     
============================================
+ Hits          18955    18975      +20     
+ Misses         4036     4035       -1     
+ Partials        760      757       -3     

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[Rosander0]

This contians the URL link to documetation explaining the longest common substring algorithm
https://www.geeksforgeeks.org/dsa/longest-common-substring-dp-29/

[prashantpiyush1111]

Hey @Rosander0, good work on the implementation! The DP approach is correct
and test coverage is solid. Just a few things to fix before this can be merged:

1. Reference URL missing from source file
Contributing guidelines require a Wikipedia/reference link directly in the
Javadoc of the implementation file, not just in the PR comments.

@see Wikipedia: Longest Common Substring

2. Fragile edge case
When no common substring exists, the method returns "" by coincidence (substring(0,0)).
Please add an explicit guard:

if (maxLength == 0) {
return "";
}

3. Tie-breaking behavior undocumented
The "first longest wins" contract should be documented in the method's
Javadoc in the implementation file, not just in the test comment.

4. Branch needs cleanup
There are 11 commits with duplicates and unnecessary merge commits:

• "fix: resolve build errors and formatting" appears twice
• "fix: add blank line before imports" appears twice
• 2 unnecessary merge commits

Please squash into 1 clean commit:

git rebase -i HEAD~11

Final commit message should be:
"feat: add LongestCommonSubstring implementation"

5. Minor
Typo in PR title: "Impleamentation" → "Implementation"

---

Overall the logic is clean, all CI checks are passing and test coverage
is 100%. Fix these small things and this should be good to go!

[prashantpiyush1111]

All requested changes have been addressed — reference URL added in Javadoc, explicit maxLength == 0 guard added, and tie-breaking behavior documented in method's Javadoc. LGTM!