feat: add Bell Numbers algorithm using Aitken's Array

[singhc7]

Description

This PR adds the Bell Numbers algorithm (BellNumbers.java), which counts the number of possible partitions of a set.

Implementation Details:

• Implements the Bell Triangle (Aitken's Array) method.

• Complexity: $O(n^2)$ Time / $O(n^2)$ Space.

• Safety: Added a guard clause to throw IllegalArgumentException for n > 25, as $B_{26}$ exceeds the range of a 64-bit Java long.

• Testing: Added BellNumbersTest.java with 6 unit tests covering:

  • Base cases ($n=0, 1$)
  • Standard cases
  • Maximum long capacity ($n=25$)
  • Overflow/Negative input handling

• 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 79.06%. Comparing base (79cdb98) to head (f3a25a0).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7219      +/-   ##
============================================
+ Coverage     79.05%   79.06%   +0.01%     
- Complexity     6946     6952       +6     
============================================
  Files           779      780       +1     
  Lines         22908    22921      +13     
  Branches       4502     4506       +4     
============================================
+ Hits          18110    18123      +13     
  Misses         4077     4077              
  Partials        721      721              

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