Fix UI freeze during driverless PPD validation

[SinghCod3r]

Summary

Driverless PPD validation can block the GTK main loop while getServerPPD() performs a CUPS IPP request. This causes the s-c-p UI to freeze temporarily while searching for drivers.

This change moves driverless PPD validation off the GTK main thread and resumes the printer-installation flow when validation completes.

Changes

• Run driverless PPD validation asynchronously instead of blocking the GTK main loop.
• Use a separate CUPS connection for the background validation request.
• Resume the installation state machine through the GTK main loop after validation completes.
• Ignore stale completion callbacks when the dialog has been destroyed or a newer validation is active.
• Preserve the validated driverless PPD in the existing cache/catalog flow.
• Return INSTALL_RESULT_OPS_PENDING while asynchronous validation is in progress.
• Update test_newprinter.py to model the asynchronous validation/completion flow without globally monkeypatching production methods.
• Remove the obsolete test patching approach.

Testing

Focused tests:

pytest -q test_newprinter.py test_PhysicalDevice.py
97 passed

Full test suite:

pytest -q
111 passed, 1 failed

The remaining failure is in test_ppds.py and is caused by the local CUPS/PPD catalog mismatch for the Epson Stylus D78 and Canon iP3000. This failure is pre-existing and is unrelated to these changes.

Also verified:

• git diff --check passes.
• The existing driverless printer installation flows continue to pass.
• The GUI no longer requires a second Forward click after asynchronous driverless validation completes.
• The driver-selection flow no longer briefly exposes the Apply button when returning to the driver-selection page.

Supersedes #479

This PR contains the same implementation previously submitted in #479. The previous PR was closed after its source branch was deleted. The implementation itself has been preserved and is now submitted from fix/driverless-ppd-validation-async.