fix(ci): force PySide6 stubs in updater test

- replace any preloaded PySide6 modules with local stubs before importing updater_gui

- prevent headless linux failures from libGL dependency during unit tests

tests/test_updater_gui.py

@@ -7,10 +7,8 @@ import types
 
 
 def _install_pyside6_stubs():
-    if "PySide6" in sys.modules:
-        return
-
     pyside6_module = types.ModuleType("PySide6")
+    pyside6_module.__path__ = []  # treat as package
     qtcore_module = types.ModuleType("PySide6.QtCore")
     qtgui_module = types.ModuleType("PySide6.QtGui")
     qtwidgets_module = types.ModuleType("PySide6.QtWidgets")
@@ -63,6 +61,11 @@ def _install_pyside6_stubs():
     qtwidgets_module.QPushButton = _Widget
     qtwidgets_module.QHBoxLayout = _Widget
 
+    # Force stubs even if real PySide6 was imported earlier in the process.
+    for mod_name in list(sys.modules.keys()):
+        if mod_name == "PySide6" or mod_name.startswith("PySide6."):
+            del sys.modules[mod_name]
+
     sys.modules["PySide6"] = pyside6_module
     sys.modules["PySide6.QtCore"] = qtcore_module
     sys.modules["PySide6.QtGui"] = qtgui_module