Add Factorial algorithm using recursion

[Ayush41]

Description

This PR adds an implementation of the Factorial algorithm using recursion under the com.thealgorithms.recursion package.

✅ Summary:

• Added Factorial.java to demonstrate factorial calculation using recursion.
• Added corresponding test file FactorialTest.java for unit testing.
• Follows the repository’s structure, documentation style, and naming conventions.
• Includes exception handling for negative inputs.

🧠 Algorithm Overview:

The factorial of a non-negative integer n is defined as:
Time Complexity: O(n)
Space Complexity: O(n) (recursive stack)

• 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 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 77.11%. Comparing base (14a23b7) to head (d369bd5).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6738      +/-   ##
============================================
- Coverage     77.11%   77.11%   -0.01%     
+ Complexity     6111     6110       -1     
============================================
  Files           719      719              
  Lines         20437    20437              
  Branches       3952     3952              
============================================
- Hits          15761    15760       -1     
  Misses         4079     4079              
- Partials        597      598       +1     

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