[utils] `toWei` accepts fractional wei and returns wrong values

Carried over from https://github.com/MetaMask/utils/pull/303, which was open when `@metamask/utils` moved into core.

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

```js
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`:

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

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.

Start with packages/utils/src/unitsConversion.ts and compare the behavior with the original pull request linked in the issue. Add regression coverage for fractional wei values such as 0.5, 1.5, and 0.9, then run the relevant utils tests and confirm fractional wei is rejected while whole wei remains unchanged.

Written by the indexing model from the issue text.