Export the Machine Learning Scorer

### Task Summary

The Machine Learning Scorer reads one label through two columns, the actual and the predicted, and reports the metrics a task admits. The export says which task it is scoring the same way the operator does, so the two paths select the same metrics from the same pair.

Two behaviour fixes travel with it. It reports the settings it cannot score rather than failing deep inside the generated Python, and it scores the rows an empty cell leaves usable rather than ending the run on one.

Step 19 of 27 in #8325. It needs #8327 for the trait. Those two behaviour fixes are filed separately as #8302 and #8058 and close with this work.