fix: logging KeyError with reserved 'filename' key, pytest return-not-none warnings

- api_server.py: rename 'filename' to 'file_name' in upload_file log extra
  dict to avoid collision with Python logging's reserved LogRecord attribute
- test_e2e_ocr.py: replace return statements with assert in test functions
  to fix PytestReturnNotNoneWarning

api_server.py

@@ -361,7 +361,7 @@ async def upload_file(file: UploadFile = File(...), session_id: str = ""):
     }
 
     _api_log.info("文件上传", extra={
-        "file_id": file_id, "filename": safe_name, "size": len(content),
+        "file_id": file_id, "file_name": safe_name, "size": len(content),
     })
 
     return {