testing: improve SortStackTest

[alxkm]

This PR enhances the test coverage for the SortStack.sortStack() method by introducing a variety of meaningful and edge case scenarios. The following test cases were added:

Edge Orderings:

• testSortReverseSortedStack – fully reversed input
• testSortTwoElements – minimal stack with two unsorted values
• testSortTwoElementsAlreadySorted – minimal stack with sorted values

Duplicates & Uniform Values:

• testSortWithAllSameElements – all elements are identical
• testSortWithManyDuplicates – mixed duplicates across several values

Negative and Boundary Values:

• testSortWithNegativeNumbers – mix of negative and positive integers
• testSortWithAllNegativeNumbers – only negative integers
• testSortWithZero – includes zero
• testSortStackWithMinAndMaxValues – tests with Integer.MIN_VALUE and Integer.MAX_VALUE

General Robustness:

• testSortLargerStack – stack with 10 varied elements
• testOriginalStackIsModified – asserts that the original stack reference is sorted in-place

• 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 74.86%. Comparing base (31bf130) to head (c094ea8).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #6422   +/-   ##
=========================================
  Coverage     74.86%   74.86%           
- Complexity     5482     5483    +1     
=========================================
  Files           682      682           
  Lines         19012    19012           
  Branches       3670     3670           
=========================================
+ Hits          14233    14234    +1     
  Misses         4224     4224           
+ Partials        555      554    -1     

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