test(icons): guard against the material icon segfault in cyclic garbage

Runs the device manager's icon caches through cyclic garbage collection in a child process. It segfaults (SIGSEGV in the QIcon destructor) with bec_qthemes releases that lack the icon engine ownership fix, and passes once bec_widgets requires a fixed bec_qthemes.

tests/unit_tests/test_material_icon_cycle_exit_crash.py

@@ -0,0 +1,62 @@
+"""
+F18: segfault in the QIcon destructor when material icons held by a widget are collected as
+cyclic garbage (seen at interpreter exit after DeviceManagerWidget or BECMainApp tests).
+
+The crash happens in a child process so that it turns into a failing test instead of killing
+pytest. The child uses the real icon dictionaries of the device manager components, which are
+built as dict literals (icons first, dict afterwards) - the order that makes the garbage
+collector clear the icon engine before its QIcon.
+"""
+
+import os
+import subprocess
+import sys
+import textwrap
+
+import pytest
+
+_SCRIPT = textwrap.dedent("""
+    import faulthandler
+    import gc
+    import os
+
+    faulthandler.enable(all_threads=False)
+    os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
+
+    from qtpy.QtWidgets import QApplication
+
+    app = QApplication([])
+
+    from bec_widgets.utils.colors import get_accent_colors
+    from bec_widgets.widgets.control.device_manager.components.ophyd_validation.ophyd_validation_utils import (
+        get_validation_icons,
+    )
+
+
+    class IconOwner:
+        '''Stands in for a widget that caches icons and ends up in a reference cycle.'''
+
+
+    # Icons first, owner afterwards: the collector then visits every icon engine before the
+    # QIcon and the dicts that hold it, as it did for the DeviceTable/OphydValidation icon caches
+    # at interpreter exit.
+    icons = get_validation_icons(get_accent_colors(), (18, 18))
+    owner = IconOwner()
+    owner._icons = icons
+    owner.cycle = owner
+    del owner, icons
+    gc.collect()
+    print("collected", flush=True)
+    """)
+
+
+@pytest.mark.timeout(180)
+def test_material_icons_in_cyclic_garbage_do_not_crash():
+    """Collecting a cycle that holds material QIcons must not segfault."""
+    env = dict(os.environ, QT_QPA_PLATFORM="offscreen")
+    proc = subprocess.run(
+        [sys.executable, "-c", _SCRIPT], capture_output=True, text=True, timeout=150, env=env
+    )
+    assert (
+        proc.returncode == 0 and "collected" in proc.stdout
+    ), f"child exited with {proc.returncode}\n{proc.stdout[-2000:]}\n{proc.stderr[:4000]}"