multipleOf method
Returns true if input is a multiple of divisor.
A divisor of zero is treated as a schema-error guard: the spec requires
multipleOf values to be strictly greater than zero, so a zero divisor
returns false rather than throwing.
Implementation
bool multipleOf(num input, num divisor) {
if (divisor == 0) return false;
// Compute the quotient and check that its fractional part is negligibly
// small, guarding against IEEE-754 rounding in decimal arithmetic.
final quotient = input / divisor;
return (quotient - quotient.roundToDouble()).abs() < _epsilon;
}