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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