BUG: Can't include nested Headings in Directives

### Describe the bug

You can't have nested headings within exercise directives such as

````
```{exercise-start}
:label:
```

## Heading 2

```{exercise-end}
```

````

this causes the nodes not be correctly collected and parsed in the `AST`



### Reproduce the bug

See above example

### List your environment

_No response_