fix: round zeta BigNumber digit count to avoid non-integer factorial

[xianjianlf2]

Problem

The BigNumber branch of zeta derived its digit count from Math.abs(Math.log10(config.relTol)). When relTol is not an exact power of ten (e.g. 5e-8, or 1e-320 where floating-point rounding makes log10 non-integer), this produced a non-integer digit count. That value eventually reached factorial/gamma as a non-integer BigNumber, throwing Integer BigNumber expected.

Fix

Round the derived value with Math.round(...) so the digit count is always an integer, regardless of floating-point noise in Math.log10.

-        // relTol is for example 1e-12. Extract the positive exponent 12 from that
-        return Math.abs(Math.log10(config.relTol))
+        // relTol is for example 1e-12. Extract the positive exponent 12 from that.
+        // Round to guard against floating point errors in Math.log10 for very
+        // small relTol like 1e-320, which would otherwise yield a non-integer
+        // digit count (see https://github.com/josdejong/mathjs/issues/3532).
+        return Math.round(Math.abs(Math.log10(config.relTol)))

Testing

Added a unit test in test/unit-tests/function/special/zeta.test.js that configures BigNumber mode with a relTol whose base-10 logarithm is non-integer (5e-16) and asserts that zeta(3) no longer throws and returns the expected value.

Closes #3532