Add controller for selecting a dataset file

### Feature Summary

Step 6 of the operator-demo video generator: add a controller for the dataset-selection flow, so source operators can point at a real file and produce output.

Source operators read from a dataset rather than from an upstream operator. Their file is chosen through a modal that picks a dataset, a version, and a file, and only that flow sets a usable file reference on the operator.

### Proposed Solution or Design

Add a new `DatasetControllerBuilder` to the Playwright layer. It drives the file-selection modal used by source operators.

```
new DatasetControllerBuilder(ctx)
.datasetName(dataset.name)
.datasetVersion(dataset.version)
.file("movies.csv")
.execute()
```

The controller handles:
- opening the file-selection modal,
- selecting the dataset by name,
- selecting the required dataset version,
- selecting the file and confirming the choice,
- waiting for the operator to load the reference and infer its schema.

File selection must go through the modal because it sets the full dataset URI on the operator. Directly filling the form field only sets a file name and can overwrite that URI, causing the operator to fail at runtime.

Frontend hooks:
Add stable `data-testid` attributes for the modal controls where needed, and extend the guard test accordingly.

### Affected Area

_No response_