refactor: Enhance docs, add tests in `LeftistHeap`

[Hardvan]

trafficstars

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] 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 :white_check_mark:

Project coverage is 70.79%. Comparing base (709d977) to head (80d1ebd).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5982      +/-   ##
============================================
+ Coverage     70.76%   70.79%   +0.02%     
- Complexity     4859     4861       +2     
============================================
  Files           650      650              
  Lines         17533    17533              
  Branches       3379     3379              
============================================
+ Hits          12407    12412       +5     
+ Misses         4660     4656       -4     
+ Partials        466      465       -1     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.