testing: improve QueueByTwoStacksTest

[alxkm]

• Broader input variety including multiple enqueue/dequeue patterns.
• Better validation of size tracking and exception behavior.
• Simulated real-world usage patterns and edge cases.

• 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.
⚠️ Please upload report for BASE (master@7c2af29). Learn more about missing BASE report.
⚠️ Report is 328 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #6416   +/-   ##
=========================================
  Coverage          ?   74.97%           
  Complexity        ?     5533           
=========================================
  Files             ?      686           
  Lines             ?    19214           
  Branches          ?     3708           
=========================================
  Hits              ?    14406           
  Misses            ?     4247           
  Partials          ?      561           

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