Fix #1: Implement fibonacci with edge-case ValueError handling

[dogthebughunter]

Description

Fixes #1

Solution Overview

• Implemented fibonacci(n) in src/math_utils.py using an $O(n)$ iterative calculation.
• Added input validation raising ValueError('n must be non-negative') when $n < 0$.
• Added comprehensive unit test coverage in tests/test_math_utils.py for standard numbers (0, 1, 2, 3, 5, 10, 20) and negative numbers (-1, -10).

Verification & Test Evidence

All unit tests passed with 0 failures or regressions:

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_base_and_normal_cases PASSED    [ 50%]
tests/test_math_utils.py::test_fibonacci_negative_edge_cases 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 0.07s ===============================

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