[eliotwrobson]

The Lint job ran ruff with continue-on-error, so the 131 errors on main never failed it. This fixes them and removes continue-on-error.

• Scenario fixtures: files under tests/scenario_root/ stand in for student code. They use names the sandbox injects, import inside functions to test import restrictions, and evaluate bare expressions, so F821, PLC0415 and B018 are ignored there. That covers most of the errors.
• Excluded: the three files that test syntax-error handling, since they can't be parsed, and the fixture notebooks, since ruff format rewrites a #grade cell tag as # grade.
• json_utils.py: PLC0415 is ignored, because numpy and pandas aren't in the lite image.
• Everything else is fixed in code: a from None on a re-raise, split compound asserts, zip(strict=True), an isinstance check, import order, and noqa on the intentional exec and dill.loads. ruff format also joined one line in config.py.
• Removed PT004 from the ignore list, because ruff no longer has that rule.

There's no branch protection on main, so the Lint job isn't a required check yet. That has to be set in the repository settings.

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