Quickselect median of medians

[lukmanu1]

This PR adds a deterministic QuickSelect algorithm using the Median of Medians method in the divideandconquer package.

• Fully static and final class
• Finds the kth smallest element in O(n) worst-case time
• Includes JavaDoc documentation
• Includes JUnit 5 tests for multiple cases (duplicates, negatives, small arrays, invalid inputs)

Closes #6660

• 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

❌ Patch coverage is 93.18182% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.79%. Comparing base (9484c7e) to head (dff12ad).

Files with missing lines	Patch %	Lines
...s/divideandconquer/QuickSelectMedianOfMedians.java	93.18%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6661      +/-   ##
============================================
+ Coverage     75.75%   75.79%   +0.04%     
- Complexity     5772     5791      +19     
============================================
  Files           703      704       +1     
  Lines         19777    19821      +44     
  Branches       3832     3842      +10     
============================================
+ Hits          14982    15024      +42     
- Misses         4215     4216       +1     
- Partials        580      581       +1     

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[DenizAltunkapan]

@lukmanu1 What's the difference between your implementation and the existing QuickSelect? Both appear to implement the Quickselect algorithm using the Median of Medians pivot selection. Could you clarify what improvements, differences in complexity, or design goals your version introduces?