Clean up Qt crash handlers before shutdown

[dovvnloading]

Summary

• Add an idempotent crash-handler teardown routine.
• Remove the Qt message callback and disable faulthandler before PySide teardown.
• Register cleanup on QApplication.aboutToQuit.
• Add regression coverage for restoring process-global handlers.

Root cause

The CI bundle showed the Qt test suite completing all 482 tests and then exiting with status 1. Earlier main-branch logs also showed a late QObject::eventFilter() Shiboken exception during interpreter shutdown. Crash reporting installed global Python/Qt handlers but never removed them, allowing callbacks to run after Qt wrappers were being destroyed.

Validation

• Focused crash suite: 22 passed
• Full suite: 483 passed
• Python compilation: passed