Sort character by frequency

[lieutenant-Rohit]

Added SortCharacterByFrequency algorithm using HashMap and Priority Queue

• Counts frequency of characters using HashMap
• Uses a max-heap (PriorityQueue) to sort characters by frequency
• Includes JUnit test cases for validation
• Code formatted using clang-format and follows project conventions

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.55%. Comparing base (b3e31b5) to head (732517c).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7391      +/-   ##
============================================
+ Coverage     79.53%   79.55%   +0.02%     
- Complexity     7176     7184       +8     
============================================
  Files           798      799       +1     
  Lines         23467    23486      +19     
  Branches       4617     4621       +4     
============================================
+ Hits          18665    18685      +20     
  Misses         4055     4055              
+ Partials        747      746       -1     

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