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().

perception/provider.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import logging
 from abc import ABC, abstractmethod
 from typing import Any
 
@@ -9,6 +10,8 @@ from perception.scene_builder import build_scene as build_fused_scene
 from perception.scene_builder import infer_png_size
 from perception.ui_parser import parse_ui_tree
 
+logger = logging.getLogger(__name__)
+
 
 class PerceptionProvider(ABC):
     @abstractmethod
@@ -22,10 +25,17 @@ class DefaultPerceptionProvider(PerceptionProvider):
 
     def build_scene(self, screenshot: bytes, tree: Any) -> Scene:
         width, height = infer_png_size(screenshot)
+        try:
+            ui_elements = parse_ui_tree(tree)
+        except Exception:
+            logger.warning(
+                "UI tree parsing failed; continuing without UI tree", exc_info=True
+            )
+            ui_elements = []
         return build_fused_scene(
             screen_width=width,
             screen_height=height,
-            ui_elements=parse_ui_tree(tree),
+            ui_elements=ui_elements,
             ocr_elements=run_ocr(screenshot, engine=self._ocr_engine),
         )