highgui test: force offscreen Qt platform to avoid CI crash

CI (Ubuntu ARM64 x2) showed the Python test suite aborting the whole
process with exit code 134: on a Qt-enabled build, namedWindow() tried
the "xcb" platform plugin, found no display (the Python test step,
unlike the C++ gtest step, does not run under xvfb-run), and Qt
aborted the process instead of raising a catchable exception -- so the
existing try/except around namedWindow never got a chance to run.

Fix: set QT_QPA_PLATFORM=offscreen (only if not already set) before
any GUI call. "offscreen" is the platform plugin Qt ships specifically
for headless/CI use, and is listed as available in the failing CI
job's own log output.

Verified locally: built opencv_python3 with WITH_QT=ON (real Qt6, not
the WITH_QT=OFF build used in the initial verification), and confirmed
that in a clean environment (no DISPLAY, no QT_QPA_PLATFORM preset),
namedWindow + saveWindowParameters + loadWindowParameters all succeed
with this fix, where they would previously depend on a native display
backend being reachable.

modules/highgui/misc/python/test/test_window_parameters.py

@@ -1,6 +1,16 @@
 #!/usr/bin/env python
 from __future__ import print_function
 
+import os
+
+# Force Qt's headless-safe platform plugin when nothing else is already
+# configured. Without this, namedWindow() on a Qt-enabled build with no
+# display server available (e.g. CI workers, which do not run the Python
+# test suite under a virtual framebuffer) aborts the whole process instead
+# of raising a catchable exception -- so this must be set *before* the
+# first GUI call, and a plain try/except around namedWindow is not enough.
+os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
+
 import cv2 as cv
 
 from tests_common import NewOpenCVTests