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_describe_screen.py

@@ -0,0 +1,82 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+from core.errors import DriverError
+from core.models import Bounds
+from device.manager import DeviceManager
+from driver.base import Driver
+from perception.ocr import OCRBox
+from tools.describe_screen import describe_screen
+
+PNG_10X20 = b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x0a\x00\x00\x00\x14"
+
+
+class FakeOCREngine:
+    def __init__(self, boxes: list[OCRBox]) -> None:
+        self._boxes = boxes
+
+    def extract(self, image: bytes | str | Path) -> list[OCRBox]:
+        return self._boxes
+
+
+class TreeFailingDriver(Driver):
+    def connect(self) -> None:
+        return None
+
+    def disconnect(self) -> None:
+        return None
+
+    def screenshot(self) -> bytes:
+        return PNG_10X20
+
+    def tap(self, x: float, y: float) -> None:
+        return None
+
+    def swipe(
+        self,
+        start_x: float,
+        start_y: float,
+        end_x: float,
+        end_y: float,
+        duration_ms: int = 500,
+    ) -> None:
+        return None
+
+    def input(self, text: str) -> None:
+        return None
+
+    def launch(self, app_id: str) -> None:
+        return None
+
+    def terminate(self, app_id: str) -> None:
+        return None
+
+    def tree(self):
+        raise DriverError("ui tree retrieval failed")
+
+    def home(self) -> None:
+        return None
+
+    def lock(self) -> None:
+        return None
+
+    def unlock(self) -> None:
+        return None
+
+
+def test_describe_screen_degrades_to_ocr_only_when_ui_tree_unavailable() -> None:
+    manager = DeviceManager()
+    manager.register_device("phone-1", lambda: TreeFailingDriver())
+    manager.connect("phone-1")
+    fake_box = OCRBox(text="Search", bounds=Bounds(1, 2, 4, 4), confidence=0.9)
+    fake_engine = FakeOCREngine([fake_box])
+
+    scene = describe_screen(
+        "phone-1",
+        manager=manager,
+        ocr_engine=fake_engine,  # type: ignore[arg-type]
+    )
+
+    assert [element.text for element in scene.elements] == ["Search"]
+    assert all(element.source == "ocr" for element in scene.elements)

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),