panic: assertion `left == right` failed

### Summary

ty crashes when checking this (fuzzed) code:

```python
x: int

type x[T] = x[T, U]
```

```
Panicked at crates/ty_python_semantic/src/types/infer/builder.rs:11296:22 when checking `/home/user/foo.py`: `assertion `left == right` failed
  left: Some(Dynamic(Unknown))
 right: None`
```

### Version

ty 0.0.1-alpha.29