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().

tests/test_perception_provider.py

@@ -9,13 +9,7 @@ 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
 
-PNG_10X20 = (
-    b"\x89PNG\r\n\x1a\n"
-    b"\x00\x00\x00\r"
-    b"IHDR"
-    b"\x00\x00\x00\x0a"
-    b"\x00\x00\x00\x14"
-)
+PNG_10X20 = b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x0a\x00\x00\x00\x14"
 
 TREE_XML = """
 <AppiumAUT x="0" y="0" width="10" height="20">
@@ -57,6 +51,27 @@ def test_null_perception_provider_infers_zero_size_for_non_png() -> None:
     assert scene == Scene(width=0, height=0, elements=[])
 
 
+def test_default_perception_provider_degrades_when_ui_tree_parsing_fails() -> None:
+    fake_boxes = [
+        OCRBox(text="Search", bounds=parse_ui_tree(TREE_XML)[0].bounds, confidence=0.9),
+    ]
+    fake_engine = FakeOCREngine(fake_boxes)
+    provider = DefaultPerceptionProvider(ocr_engine=fake_engine)  # type: ignore[arg-type]
+
+    scene = provider.build_scene(PNG_10X20, "<unclosed-tag")
+
+    width, height = infer_png_size(PNG_10X20)
+    expected = build_fused_scene(
+        screen_width=width,
+        screen_height=height,
+        ui_elements=[],
+        ocr_elements=[fake_boxes[0].to_scene_element("ocr-000")],
+    )
+
+    assert scene == expected
+    assert scene.elements == [fake_boxes[0].to_scene_element("ocr-000")]
+
+
 def test_default_perception_provider_wires_through_scene_builder() -> None:
     fake_boxes = [
         OCRBox(text="Search", bounds=parse_ui_tree(TREE_XML)[0].bounds, confidence=0.9),