[utils] toWei accepts fractional wei and returns wrong values #10209

Carried over from MetaMask/utils#303, which was open when @metamask/utils moved into core.

toWei silently accepts fractional wei and returns the wrong number. Confirmed against main today:

import { toWei } from '@metamask/utils';

toWei('0.5', 'wei'); // 5n
toWei('1.5', 'wei'); // 6n
toWei('0.9', 'wei'); // 9n
toWei('1', 'wei');   // 1n

Wei is the base unit, so anything with a fractional part should be rejected rather than scaled.

The cause is in packages/utils/src/unitsConversion.ts:

const unitLengths = Object.fromEntries(
  Object.entries(unitMap).map(([key, value]) => [key, value.length - 1 || 1]),
) as Record<EthereumUnit, number>;

Wei's raw value is '1', so value.length - 1 is 0, which is the correct precision for the base unit. The || 1 fallback then turns that 0 into 1, so the fraction gets treated as one decimal place of a larger unit.

The original PR has the fix plus tests.