test(ocr): add resilience tests for tiny image and unexpected exception propagation

Add test for 1×1 image (sub-tile-size) resilience and narrow preprocess_page
fallback from except Exception to (cv2.error, ValueError, MemoryError) so
programming errors propagate instead of being silently swallowed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ocr-service/test_preprocessing.py

@@ -48,17 +48,35 @@ class TestPreprocessPage:
 
         assert l_mean_after > l_mean_before
 
+    def test_does_not_crash_on_sub_tile_size_image(self):
+        """A 1×1 image is smaller than the CLAHE tile (8×8) in both axes.
+        preprocess_page must not raise — it either succeeds or falls back silently."""
+        from preprocessing import preprocess_page
+
+        img = Image.new("RGB", (1, 1), color=(128, 100, 80))
+        result = preprocess_page(img)
+
+        assert isinstance(result, Image.Image)
+
     def test_falls_back_to_pixel_identical_original_on_cv2_error(self):
-        """When cv2 raises, preprocess_page must return the unmodified original image."""
+        """When cv2 raises a known error, preprocess_page returns the unmodified original image."""
         from preprocessing import preprocess_page
 
         img = Image.new("RGB", (80, 60), color=(123, 45, 67))
         original_pixels = list(img.getdata())
 
-        with patch("preprocessing.cv2") as mock_cv2:
-            mock_cv2.cvtColor.side_effect = RuntimeError("cv2 exploded")
-
+        with patch("preprocessing.cv2.cvtColor", side_effect=ValueError("bad input")):
             result = preprocess_page(img)
 
         result_pixels = list(result.getdata())
         assert result_pixels == original_pixels
+
+    def test_unexpected_exception_propagates(self):
+        """A RuntimeError (programming error) must propagate — not be swallowed by the cv2 fallback."""
+        from preprocessing import preprocess_page
+
+        img = Image.new("RGB", (80, 60))
+
+        with patch("preprocessing.cv2.cvtColor", side_effect=RuntimeError("unexpected")):
+            with pytest.raises(RuntimeError, match="unexpected"):
+                preprocess_page(img)