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/preprocessing.py

@@ -16,7 +16,8 @@ CLAHE_TILE_SIZE = int(os.environ.get("OCR_CLAHE_TILE_SIZE", "8"))
 def preprocess_page(image: Image.Image) -> Image.Image:
     """Apply CLAHE + grayscale + Gaussian blur to improve OCR quality on aged documents.
 
-    Falls back silently to the original image if any step fails.
+    Falls back silently to the original image if cv2, numpy, or memory errors occur.
+    Unexpected exceptions (programming errors) are allowed to propagate.
     """
     try:
         img_array = np.array(image)
@@ -40,7 +41,7 @@ def preprocess_page(image: Image.Image) -> Image.Image:
         del blurred
 
         return result
-    except Exception as e:
+    except (cv2.error, ValueError, MemoryError) as e:
         logger.warning(
             "preprocess_page failed (falling back to original): %s: %s",
             type(e).__name__,