fix(ocr): accept sender_model_path in Surya engine so non-Kurrent OCR works

main.py unifies the call to both engines and always passes
`sender_model_path` (None for non-Kurrent scripts). Surya's
extract_region_text / extract_page_blocks accepted one fewer positional
arg than Kraken's, so every guided-OCR run on a TYPEWRITER or
HANDWRITING_LATIN document raised "takes 5 positional arguments but 6
were given" and the stream returned 0 blocks / 1 skipped page.

Add an ignored `sender_model_path` kwarg to both Surya functions so the
signatures match Kraken's, and guard the regression with two signature
tests in test_engines.py that compare both engines' parameter lists.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ocr-service/engines/surya.py

@@ -33,11 +33,16 @@ def load_models():
     logger.info("Surya models loaded successfully")
 
 
-def extract_page_blocks(image, page_idx: int, language: str = "de") -> list[dict]:
+def extract_page_blocks(
+    image, page_idx: int, language: str = "de", sender_model_path: str | None = None
+) -> list[dict]:
     """Run Surya OCR on a single PIL image and return block dicts for that page.
 
+    `sender_model_path` is accepted for signature parity with the Kraken engine
+    (which uses it to select a fine-tuned HTR model) and is ignored here.
     Coordinates are normalized to [0, 1].
     """
+    del sender_model_path
     load_models()
 
     page_w, page_h = image.size
@@ -81,12 +86,22 @@ def extract_page_blocks(image, page_idx: int, language: str = "de") -> list[dict
     return blocks
 
 
-def extract_region_text(image, x: float, y: float, w: float, h: float) -> str:
+def extract_region_text(
+    image,
+    x: float,
+    y: float,
+    w: float,
+    h: float,
+    sender_model_path: str | None = None,
+) -> str:
     """Crop image to a normalized region and run Surya recognition on the crop.
 
     Used for guided OCR — skips full-page layout detection and only processes
-    the given bounding box. Coordinates are normalized to [0, 1].
+    the given bounding box. `sender_model_path` is accepted for signature
+    parity with the Kraken engine and is ignored here. Coordinates are
+    normalized to [0, 1].
     """
+    del sender_model_path
     load_models()
 
     pw, ph = image.size