test: refactor MergeSortedArrayListTest using JUnit 5 parameterized tests

[sawaleparas]

Description

This PR upgrades the existing MergeSortedArrayListTest by refactoring individual test scenarios into a unified, data-driven @ParameterizedTest.

Changes Implemented

• Consolidated multiple standard and edge-case testing loops into a clean, parameterized scenario method using @MethodSource.
• Split up the multi-assertion NullPointerException test into separate, dedicated validation methods (testMergeThrowsExceptionWhenListAIsNull, etc.) for sharper failure isolation.
• Maintained clean imports and adhered strictly to the repository's naming conventions (testMergeParameterizedScenarios).

Verification Results

All 9 test cases passed successfully locally inside IntelliJ IDEA with zero regressions.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.69%. Comparing base (978a306) to head (55b169a).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7445      +/-   ##
============================================
- Coverage     79.70%   79.69%   -0.02%     
+ Complexity     7275     7272       -3     
============================================
  Files           802      802              
  Lines         23700    23700              
  Branches       4661     4661              
============================================
- Hits          18891    18888       -3     
  Misses         4055     4055              
- Partials        754      757       +3     

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