[SaifAli-S]

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Fixes #15753

num_digits_fast() in maths/number_of_digits.py previously evaluated math.floor(math.log10(abs(n)) + 1). For integers where abs(n) >= 10**16 - 1 (exceeding IEEE 754 53-bit float mantissa precision), floating-point rounding near powers of ten caused an off-by-one error (e.g. num_digits_fast(10**16 - 1) evaluated to 17 instead of 16).

This fix verifies the logarithmic estimate against integer powers of ten (10 ** (digits - 1) and 10 ** digits), correcting any floating-point rounding discrepancies while maintaining $O(1)$ fast calculation speed. Added regression doctests covering boundary values ($10^{16}-1$, $10^{16}$, $10^{50}-1$, and negative integers).

Checklist

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• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
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