fix(maths): raise documented ValueError for negative inputs in res()

[amir-rezaei]

Description

This PR resolves issue #14930 by adding the documented x < 0 validation check in res() in maths/largest_of_very_large_numbers.py.

Details

• Raises ValueError("expected a positive input") when x < 0, aligning the code with the existing doctest contract.

[algorithms-keeper]

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