Fix PackageKit D-Bus lifecycle

[SinghCod3r]

What does this PR do?

Fixes the PackageKit private D-Bus connection lifecycle in the printer setup flow.

PackageKit fix

The private PackageKit D-Bus connection was being closed from the worker thread while GLib could still have pending messages for that connection.

This change:

• disables exit_on_disconnect
• keeps the private D-Bus connection alive until the GLib completion callback
• closes the connection from the completion callback
• falls back to querying CUPS when the PackageKit request fails

Regression test

Adds a regression test covering the failure path to verify that:

• the private D-Bus connection is created with private=True
• exit_on_disconnect is disabled
• the connection is passed to the GLib completion callback
• the connection is closed from the completion callback
• CUPS fallback is triggered after the PackageKit failure

Testing

• PackageKit lifecycle regression test passes.
• Full test_newprinter.py test suite passes.
• Manually tested the Xerox B235 printer flow.
• PackageKit failure now falls back to CUPS without reproducing the previous segmentation fault.