driver.tree() failures (WDA/Appium session errors) previously raised
uncaught, killing describe_screen() before OCR ever ran. Malformed
tree content (invalid XML) had the same problem inside
parse_ui_tree(). Both are now caught and logged, falling back to an
empty ui_elements list so the scene degrades to OCR-only, mirroring
the existing OCR-failure fallback in run_ocr().
Existing tests only exercised perception.scene_builder.build_scene()
directly or substituted a FakePerceptionProvider, so the real provider
port classes had no direct coverage. Both were already correct on
manual inspection; this closes the coverage gap.
openspec: perception-provider capability, archived change device-agent-runtime-foundation
q792602257