fix(minifier): coerce omitted `indexOf` search argument (#26149)

## Summary

- Apply `ToString(undefined)` when `String.prototype.indexOf` or `lastIndexOf` is called without a search argument.
- Keep folding strings without the resulting `"undefined"` substring to `-1`.
- Add focused helper and minifier regression coverage.

```js
"undefined".indexOf();     // 0
"undefined".lastIndexOf(); // 0
"abcdef".indexOf();        // -1
"abcdef".lastIndexOf();    // -1
```