🧪 Add test for rust design_matrix json error path

[edithatogo]

🎯 What: Adds missing test coverage for _design_matrix_with_rust when _coerce_rows_for_rust raises a ValueError.
📊 Coverage: Tests that _design_matrix_with_rust gracefully returns None instead of propagating the ValueError or crashing.
✨ Result: Improves test coverage and ensures the fallback behavior works correctly when coercion fails.

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PR created automatically by Jules for task 13688049004501536472 started by @edithatogo

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🎉 Welcome @edithatogo! Thank you for your first pull request to mars! We're excited to have you as a contributor. Our team will review your PR soon. In the meantime, please ensure all CI checks pass.