Only recommend validated driverless PPDs

[SinghCod3r]

Summary

CUPS can return driverless: PPDs for printers that advertise driverless support through DNS-SD, even when generating the driverless PPD for that printer actually fails.

This change prevents those unvalidated driverless PPDs from being offered as drivers.

What changed

• Exclude driverless and driverless-fax PPDs from the CUPS PPD catalog.
• Validate the driverless PPD before using it.
• Add the validated driverless PPD back to the catalog with the printer's actual identity.
• If validation fails, keep the driverless PPD out of driver selection and use the normal driver-selection flow.
• Keep the validated driverless PPD available when the user chooses a different driver.

Testing

Tested with the Xerox B235 simulator:

• Working driverless printer: driverless PPD is validated and recommended.
• Broken driverless printer: driverless PPD validation fails and a normal legacy driver is offered instead.
• Manual driver selection continues to work after the fallback.

Automated tests:

• pytest -q test_newprinter.py — 65 passed
• python3 -m py_compile newprinter.py — passed
• git diff --check — passed