fix(perception): degrade to OCR-only when UI tree is unavailable

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().

tools/describe_screen.py

@@ -1,11 +1,15 @@
 from __future__ import annotations
 
+import logging
+
 from core.models import Scene
 from device.manager import DeviceManager
 from perception.ocr import PaddleOCREngine
 from perception.provider import DefaultPerceptionProvider, PerceptionProvider
 from tools._device import get_driver
 
+logger = logging.getLogger(__name__)
+
 
 def describe_screen(
     device_id: str | None = None,
@@ -16,6 +20,12 @@ def describe_screen(
 ) -> Scene:
     driver = get_driver(device_id, manager=manager)
     screenshot = driver.screenshot()
-    raw_tree = driver.tree()
+    try:
+        raw_tree = driver.tree()
+    except Exception:
+        logger.warning(
+            "UI tree retrieval failed; continuing without UI tree", exc_info=True
+        )
+        raw_tree = None
     provider = perception_provider or DefaultPerceptionProvider(ocr_engine=ocr_engine)
     return provider.build_scene(screenshot, raw_tree)