Fix a crash in preview style with assert + parenthesized string. (#3415)

The bug is in the `get_leaves_inside_matching_brackets` on the third line below:

```python
assert xxxxxxxxx.xxxxxxxxx.xxxxxxxxx(
    xxxxxxxxx
).xxxxxxxxxxxxxxxxxx(), (
    "xxx {xxxxxxxxx} xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
)
```

Including the invisible paren, third line is `).xxxxxxxxxxxxxxxxxx()), (`, that it has a matched pair then an unmatched closing paren afterwards. This PR ensures the returned leaves are actually matched.

Fixes #3414.