panic: `no entry found for key` with fuzzed slice expression

### Summary

ty crashes when checking this fuzzed code:

```python
arr[::[x for *b in y for (b: _
```

```
error[panic]: Panicked at crates/ty_python_semantic/src/semantic_index/ast_ids.rs:35:22 when checking `/home/user/a.py`: `no entry found for key`
```

### Version

ty 0.0.8