refactor: `MaximumSumOfDistinctSubarraysWithLengthK`

[alxkm]

Class cleanup. Readability improvements. Added tests.

• [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 50.78%. Comparing base (cd38531) to head (b1f4568).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5433 +/- ## ============================================ - Coverage 50.79% 50.78% -0.01% Complexity 3139 3139 ============================================ Files 523 523 Lines 15109 15108 -1 Branches 2875 2874 -1 ============================================ - Hits 7674 7673 -1 Misses 7114 7114 Partials 321 321 ```

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