test: update test to use _convert_uploadfile_to_bytes after refactoring

The test was calling _normalize_field_value expecting UploadFile→bytes conversion,
but we moved that logic to _convert_uploadfile_to_bytes as part of the separation
of concerns refactoring.

Author: yizzy

tests/functional/event_handler/_pydantic/test_openapi_validation_middleware.py

@@ -2748,7 +2748,7 @@ class DummyFieldInfo:
         annotation = Annotated[bytes, "meta"]
 
     ov = __import__("aws_lambda_powertools.event_handler.middlewares.openapi_validation", fromlist=["*"])
-    result = ov._normalize_field_value(upload, DummyFieldInfo())
+    result = ov._convert_uploadfile_to_bytes(upload, DummyFieldInfo())
     assert result == file_content