Add input validation for negative values in FibonacciSeries

[aadislnk]

• 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

What was changed

• Added input validation to handle negative input values in the fibonacci method

Why

• Prevents infinite recursion and potential stack overflow
• Improves robustness of the utility method

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.89%. Comparing base (7d51c7f) to head (dae65ce).

Files with missing lines	Patch %	Lines
...a/com/thealgorithms/recursion/FibonacciSeries.java	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7177      +/-   ##
============================================
- Coverage     78.90%   78.89%   -0.02%     
- Complexity     6857     6858       +1     
============================================
  Files           771      771              
  Lines         22686    22688       +2     
  Branches       4456     4457       +1     
============================================
- Hits          17900    17899       -1     
- Misses         4071     4073       +2     
- Partials        715      716       +1     

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[DenizAltunkapan]

Thank you for your contribution @aadislnk