Support Polars DataFrame.unpivot schema inference

Calling `DataFrame.unpivot` falls back to a bare `DataFrame`, even when its output schema can be determined statically.

```python
from typing import reveal_type

import polars as pl

df = pl.DataFrame({
"region": ["East"],
"units": [12],
"price": [1.5],
})

reveal_type(df.unpivot(index="region", on=["units", "price"])) # DataFrame
```

I would expect:

```text
DataFrame[region: String, variable: String, value: Float64]
```

Reproduced on `pyrefly 1.3.0-dev.1` with Polars 1.35.2.