testing: improve DisjointSetUnionTest

[alxkm]

Added test cases for DisjointSetUnion:

• testUnionByRankSmallerToLarger - Validates union-by-rank optimization
• testUnionByRankEqualRanks - Tests rank increment for equal-rank unions
• testLargeChainPathCompression - Verifies path compression works correctly
• testMultipleDisjointSets - Tests separate component isolation
• testEmptyValues - Tests null/empty string handling

• 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 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 75.05%. Comparing base (040d385) to head (a438779).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6440      +/-   ##
============================================
+ Coverage     75.04%   75.05%   +0.01%     
  Complexity     5540     5540              
============================================
  Files           686      686              
  Lines         19214    19214              
  Branches       3708     3708              
============================================
+ Hits          14420    14422       +2     
+ Misses         4240     4239       -1     
+ Partials        554      553       -1     

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