feat(math_utils): add fibonacci function with edge case handling (Closes #1)

[MaaBlock]

Summary of Changes

Closes #1

This PR delivers the requested fibonacci(n) implementation in src/math_utils.py with comprehensive unit tests in tests/test_math_utils.py.

Requirements Checklist

• Function name: fibonacci(n) added to src/math_utils.py
• Returns the nth Fibonacci number (0-indexed)
• Normal cases: fibonacci(0) == 0, fibonacci(1) == 1, fibonacci(5) == 5, fibonacci(10) == 55, fibonacci(20) == 6765
• Edge case: If n < 0, raises ValueError with message "n must be non-negative"
• Comprehensive test coverage added to tests/test_math_utils.py
• All 6 unit tests in test suite pass cleanly (0.08s)

Verification Evidence

$ python -m pytest tests -v
============================= test session starts =============================
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_normal_cases PASSED             [ 50%]
tests/test_math_utils.py::test_fibonacci_edge_case PASSED                [ 66%]
tests/test_string_utils.py::test_reverse_string PASSED                   [ 83%]
tests/test_string_utils.py::test_to_uppercase PASSED                     [100%]
============================== 6 passed in 0.08s ==============================

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