Fix #1: Add fibonacci function with edge case handling

[s6pa1rta3n-lab]

Summary

Implements the fibonacci(n) calculation function with strict negative input validation in src/math_utils.py and adds comprehensive test coverage in tests/test_math_utils.py.

Stipulation Compliance Checklist

• Function name: fibonacci(n)
• Added to src/math_utils.py
• Returns nth Fibonacci number (0-indexed)
• Raises ValueError with exact message "n must be non-negative" when n < 0
• Added test cases to tests/test_math_utils.py covering normal cases (0, 1, 5, 10)
• Added test cases for negative input raising ValueError
• All existing tests continue to pass (6/6 passing)

Verification

pytest tests/ -v

Output:

tests/test_math_utils.py::test_add PASSED                                [ 16%]
tests/test_math_utils.py::test_multiply PASSED                           [ 33%]
tests/test_math_utils.py::test_fibonacci PASSED                          [ 50%]
tests/test_math_utils.py::test_fibonacci_negative PASSED                 [ 66%]
tests/test_string_utils.py::test_reverse_string PASSED                   [ 83%]
tests/test_string_utils.py::test_to_uppercase PASSED                     [100%]

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