Add Fenwick tree implementation and tests

[Rohan45create]

Add FenwickTree tests, bounds checks and Javadoc

• update(-1, x) made the index 0, so i & -i stayed 0 and the loop never ended. update and query now validate the index and throw IndexOutOfBoundsException outside [0, n). Before this, out-of-range behavior was inconsistent: update(i >= n) was silently ignored, query(-1) returned 0, and other bad indices threw ArrayIndexOutOfBoundsException.
• The constructor throws IllegalArgumentException for a negative size.
• Fields are now final, Javadoc was added (with a Wikipedia link), and the misleading query comment was corrected (it returns the sum over [0, i]).
• Added FenwickTreeTest: fresh tree, single element, prefix sums, repeated updates, negative values, non-power-of-two size, zero size, invalid arguments, and a randomized comparison against naive prefix sums.

• 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 algorithms include a corresponding test class that validates their functionality.
• 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.
✅ Project coverage is 82.01%. Comparing base (f5772e3) to head (0ebac5f).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7637      +/-   ##
============================================
+ Coverage     81.95%   82.01%   +0.06%     
- Complexity     8273     8282       +9     
============================================
  Files           840      840              
  Lines         25865    25872       +7     
  Branches       5052     5053       +1     
============================================
+ Hits          21197    21219      +22     
+ Misses         3881     3865      -16     
- Partials        787      788       +1     

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[alxkm]

Thanks for the fix - the infinite loop on update is a real bug and the tests are solid. Two small things before merge: rename the test file to FenwickTreeTest.java (it currently doesn't match the class name), and add some documentation link mentioned in the checklist.