[substrait] [sqllogictest] Unsupported window frame unit: Groups

### Describe the bug

~5 sqllogictest cases are failing in Substrait round-trip mode because of the following error:
```
query failed: DataFusion error: This feature is not implemented: Unsupported window frame unit: Groups
```

### To Reproduce

```
cargo test --test sqllogictests -- --substrait-round-trip window.slt:1030
```

### Expected behavior

The sqllogictest passes in Substrait round-trip mode

### Additional context

This is part of a broader initiative to improve the reliability of Substrait-to-DataFusion logical plan conversions.
- https://github.com/apache/datafusion/issues/16248

Start by running `cargo test --test sqllogictests -- --substrait-round-trip window.slt:1030` and trace the `Unsupported window frame unit: Groups` error through the Substrait round-trip path. Use the failing `window.slt` cases as the acceptance check; done means the affected sqllogictest cases pass in Substrait round-trip mode.

Written by the indexing model from the issue text.