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

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

api/core/workflow/nodes/variable_assigner/v2/node.py

 import json
+from collections.abc import Sequence
 from typing import Any, cast
 
 from core.variables import SegmentType, Variable
         inputs = self.node_data.model_dump()
         process_data: dict[str, Any] = {}
         # NOTE: This node has no outputs
-        updated_variables: list[Variable] = []
+        updated_variable_selectors: list[Sequence[str]] = []
 
         try:
             for item in self.node_data.items:
                     value=item.value,
                 )
                 variable = variable.model_copy(update={"value": updated_value})
-                updated_variables.append(variable)
+                self.graph_runtime_state.variable_pool.add(variable.selector, variable)
+                updated_variable_selectors.append(variable.selector)
         except VariableOperatorNodeError as e:
             return NodeRunResult(
                 status=WorkflowNodeExecutionStatus.FAILED,
                 error=str(e),
             )
 
+        # The `updated_variable_selectors` is a list contains list[str] which not hashable,
+        # remove the duplicated items first.
+        updated_variable_selectors = list(set(map(tuple, updated_variable_selectors)))
+
         # Update variables
-        for variable in updated_variables:
-            self.graph_runtime_state.variable_pool.add(variable.selector, variable)
+        for selector in updated_variable_selectors:
+            variable = self.graph_runtime_state.variable_pool.get(selector)
+            if not isinstance(variable, Variable):
+                raise VariableNotFoundError(variable_selector=selector)
             process_data[variable.name] = variable.value
 
             if variable.selector[0] == CONVERSATION_VARIABLE_NODE_ID:

api/services/app_dsl_service.py

 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
 
                         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,