Automatic class string coercion is not reactive

### Describe the bug

Considering this code:

```js
class Test {
	thing = $state(0)
	toString() {
		return String(this.thing)
	}
}

const test = new Test()
```

Using `{test}` in the component template is not reactive (when `thing` changes), but `{String(test)}` is.

### Reproduction

https://svelte.dev/playground/e44b1258b39c4477a38ccb9ecaa4cd21?version=5.20.0

annoyance