test(operators): use predictions_operator in test_predictions_operator

[raashish1601]

Fixes #181

test_predictions_operator built its attribution methods with regression_operator, so predictions_operator was never exercised by this test. It also passed on main for that reason: the regression operator works on the classification model as well.

This imports predictions_operator and uses it in that test. test_regression_operator keeps using regression_operator.

Tests: pytest tests/commons/test_operators.py passes locally (Python 3.12, TensorFlow 2.20, CPU). ruff check and ruff format --check are clean on the changed file.