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

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.79%. Comparing base (f0fb971) to head (2fb1dcb).

Files with missing lines	Patch %	Lines
...in/java/com/thealgorithms/recursion/Factorial.java	71.42%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6736      +/-   ##
============================================
- Coverage     76.80%   76.79%   -0.02%     
- Complexity     6047     6051       +4     
============================================
  Files           719      720       +1     
  Lines         20342    20349       +7     
  Branches       3940     3941       +1     
============================================
+ Hits          15624    15627       +3     
- Misses         4127     4130       +3     
- Partials        591      592       +1     

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