testing: improve StackArrayTest

[alxkm]

Tests Added:
Basic Operations:

• testSingleElementOperations: Verifies stack behavior with only one element

Push/Pop Interleaving:

• testAlternatingPushPop: Tests a sequence of alternating push and pop operations to ensure LIFO consistency

Null Handling:

• testPushNullElements: Confirms support for null values, and verifies correct behavior when mixing null and non-null entries

Generic Type Support:

• testWithDifferentDataTypes: Validates correct operation when using StackArray with String type

• 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 74.99%. Comparing base (6992716) to head (a9d3063).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #6424   +/-   ##
=========================================
  Coverage     74.99%   74.99%           
  Complexity     5536     5536           
=========================================
  Files           686      686           
  Lines         19214    19214           
  Branches       3708     3708           
=========================================
  Hits          14410    14410           
  Misses         4246     4246           
  Partials        558      558           

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