Use BigInteger to prevent overflow in factorial calculation

[VanshSharma3]

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Description

Updated the Factorial.java implementation to use BigInteger instead of long.

Reason for change:
The previous implementation using long would overflow for any input $n > 20$. Since factorials grow exponentially, BigInteger is necessary to handle arbitrarily large results accurately without data loss.

Changes:

• Changed return type to BigInteger.
• Updated the iterative loop to use BigInteger.multiply().
• Added a check for negative input to throw IllegalArgumentException.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.39%. Comparing base (635d54a) to head (848f3c6).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7358      +/-   ##
============================================
- Coverage     79.40%   79.39%   -0.01%     
- Complexity     7092     7093       +1     
============================================
  Files           792      792              
  Lines         23226    23226              
  Branches       4568     4568              
============================================
- Hits          18443    18441       -2     
- Misses         4046     4047       +1     
- Partials        737      738       +1     

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