panic: `assertion failed: start.raw <= end.raw` with unclosed forward reference

### Summary

ty panics when checking this code:

```python
a:'
```

```
error[panic]: Panicked at crates/ruff_python_ast/src/nodes.rs:1657:9 when checking `/tmp/foo.py`: `assertion failed: start.raw <= end.raw`
```



### Version

ty 0.0.1-alpha.27