Use StrEnum for DoclingTaskState; chain exception

Switch DoclingTaskState to StrEnum (imported from enum) so enum members are proper string enums. Also preserve the original error context in LangflowFileService by raising the upload exception with "from e" to keep exception chaining and traceback information.

Author: edwinjosechittilappilly

src/services/docling_service.py

@@ -2,7 +2,7 @@
 import json
 import platform
 from dataclasses import dataclass
-from enum import Enum
+from enum import StrEnum
 from pathlib import Path
 from typing import Any
 
@@ -34,7 +34,7 @@ class DoclingServeError(Exception):
     """Raised when docling-serve conversion fails."""
 
 
-class DoclingTaskState(str, Enum):
+class DoclingTaskState(StrEnum):
     """Result of a single status check against Docling Serve."""
 
     PENDING = "pending"

src/services/langflow_file_service.py

@@ -541,7 +541,7 @@ async def submit_to_docling(self, filename: str, content: bytes, jwt_token: str
                 "[LF] Docling submission failed",
                 extra={"error": str(e), "filename": filename},
             )
-            raise Exception(f"Docling upload failed: {str(e)}")
+            raise Exception(f"Docling upload failed: {str(e)}") from e
 
     async def upload_and_ingest_file(
         self,