[SinghCod3r]

Summary

This change removes the remaining confirmed blocking operations from the printer setup flow and moves SMB Browse/Verify work off the GTK main thread.

The goal is to keep the printer setup UI responsive while network, package, driver, PPD, and SMB operations are running.

What changed

• Move LPD/LPR probing off the GTK thread.
• Move PackageKit printer-driver/backend installation work to worker operations.
• Move OpenPrinting PPD download, parsing, and validation off GTK.
• Move local cupstestppd validation off GTK.
• Move GPG fingerprint retrieval off GTK.
• Move remote CUPS/HTTP queue retrieval off GTK.
• Move downloadable driver installation subprocess handling off GTK.
• Move SMB Browse, Verify, and row expansion network operations to worker paths.
• Add a GTK-safe authentication bridge for interactive SMB authentication while keeping SMB network I/O in the worker.
• Add cancellation and stale-result protection for asynchronous operations.
• Preserve driverless PPD selection while adding on-demand validation when CUPS does not advertise the required driverless PPD.
• Isolate cached driverless PPDs per device/URI to prevent cross-device leakage.
• Preserve same-device driverless cache reuse.
• Add loading feedback for driverless validation when PPDs are already loaded, avoiding a period of no visible progress.

Regression coverage

The tests cover:

• asynchronous LPD, PackageKit, PPD, GPG, remote CUPS, driver installation, and SMB paths
• SMB authentication and cancellation behavior
• driverless PPD validation and cache reuse
• multi-device driverless isolation
• identical-model printers on different network hosts
• Normal Xerox / Broken Xerox driverless selection
• PPD-mode device reconstruction
• duplicate spinner prevention

Verification

All PR checks are passing.

The final change is intentionally limited to the printer setup implementation and its regression tests.

[SinghCod3r]

Closing in favor of smaller, measured fixes. SMB is out of scope for now.