qtvcp: close dbus bus when sys_notify init fails

[grandixximo]

Summary

lib/python/qtvcp/lib/sys_notify.py constructs a dbus.SessionBus
with the PyQt5 (or PyQt6) mainloop integration before probing for
org.freedesktop.Notifications. When the probe raises (e.g.
org.freedesktop.DBus.Error.ServiceUnknown because no notification
daemon owns the name on the session bus), the Python bus reference
is dropped on the exception but the mainloop layer has already
attached a QSocketNotifier to the connection's fd. That keeps the
C-level connection alive in a half-initialized state. On the next
Qt event-loop tick the queued message is dispatched via
dbus_connection_dispatch() and segfaults inside
_dbus_list_unlink because the connection's internal queue is no
longer consistent.

Closing the bus on the failure path detaches the notifier and lets
the connection be released. The Desktop Notify not available
warning continues to be the only user-visible side effect.

Repro

Ubuntu 24.04 GitHub runner with xvfb + QT_QPA_PLATFORM=offscreen
and no session-bus notification daemon. Surfaced during PR #4054
qtdragon ui-smoke; captured under gdb -batch (Thread 1 only):

#0  _dbus_list_unlink            libdbus-1.so.3
#1  _dbus_list_pop_first_link    libdbus-1.so.3
#2  _dbus_connection_unlock      libdbus-1.so.3
#4  dbus_connection_dispatch     libdbus-1.so.3
#5  ???                          dbus/mainloop/pyqt5.abi3.so
#6  QObject::event               libQt5Core.so.5
#7  QApplicationPrivate::notify_helper  libQt5Widgets.so.5
#9  QCoreApplication::notifyInternal2   libQt5Core.so.5
#10 QCoreApplicationPrivate::sendPostedEvents
#14 g_main_context_iteration     libglib-2.0.so.0
#15 QEventDispatcherGlib::processEvents
#16 QEventLoop::exec
#17 QCoreApplication::exec()     ← qtvcp.py APP.exec()

Same investigation surfaced PR #4062 (shutdown-time hal_exit
use-after-free in QPin timer). The two are independent: #4062 is
the teardown path, this is the startup path.

Test plan

• qtdragon ui-smoke pre-fix: SIGSEGV in _dbus_list_unlink on
  Ubuntu 24.04 (PR test: ui-smoke phase 2, g-code execution and endpoint check #4054 CI runs 26561722252, 26572207672)
• qtdragon ui-smoke post-fix: package-arch debian:sid passes in
  71s (matches pre-regression baseline)
• python3 -m py_compile lib/python/qtvcp/lib/sys_notify.py
  clean

Risk

The new except branch only runs when init has already failed.
bus.close() is the documented teardown for dbus.Connection;
swallowing exceptions from it is safe because the connection is
already being abandoned.