feat(sandbox) Adds download file interface in Sandbox (#3038)

* Add download interface in Sandbox

* fix

* fix

* del invalidate test

* fix

* safe download

* improve

backend/packages/harness/deerflow/sandbox/sandbox.py

@@ -39,6 +39,25 @@ class Sandbox(ABC):
         """
         pass
 
+    @abstractmethod
+    def download_file(self, path: str) -> bytes:
+        """Download the binary content of a file.
+
+        Args:
+            path: The absolute path of the file to download.
+
+        Returns:
+            Raw file bytes.
+
+        Raises:
+            PermissionError: If path traversal is detected or the path is outside
+                the allowed virtual prefix.
+            OSError: If the file cannot be read or does not exist.  Both local
+                and remote implementations must raise ``OSError`` so callers
+                have a single exception type to handle.
+        """
+        pass
+
     @abstractmethod
     def list_dir(self, path: str, max_depth=2) -> list[str]:
         """List the contents of a directory.