fix: Update variable handling in VariableAssignerNode and clean up app_dsl_service (#12672)

Signed-off-by: -LAN- <laipz8200@outlook.com>

api/services/app_dsl_service.py

@@ -1,7 +1,7 @@
 import logging
 import uuid
 from enum import StrEnum
-from typing import Optional, cast
+from typing import Optional
 from urllib.parse import urlparse
 from uuid import uuid4
 
@@ -139,15 +139,6 @@ class AppDslService:
                         status=ImportStatus.FAILED,
                         error="Empty content from url",
                     )
-
-                try:
-                    content = cast(bytes, content).decode("utf-8")
-                except UnicodeDecodeError as e:
-                    return Import(
-                        id=import_id,
-                        status=ImportStatus.FAILED,
-                        error=f"Error decoding content: {e}",
-                    )
             except Exception as e:
                 return Import(
                     id=import_id,