feat: Persist Variables for Enhanced Debugging Workflow (#20699)

This pull request introduces a feature aimed at improving the debugging experience during workflow editing. With the addition of variable persistence, the system will automatically retain the output variables from previously executed nodes. These persisted variables can then be reused when debugging subsequent nodes, eliminating the need for repetitive manual input.

By streamlining this aspect of the workflow, the feature minimizes user errors and significantly reduces debugging effort, offering a smoother and more efficient experience.

Key highlights of this change:

- Automatic persistence of output variables for executed nodes.
- Reuse of persisted variables to simplify input steps for nodes requiring them (e.g., `code`, `template`, `variable_assigner`).
- Enhanced debugging experience with reduced friction.

Closes #19735.

api/core/workflow/nodes/loop/loop_end_node.py

@@ -13,6 +13,10 @@ class LoopEndNode(BaseNode[LoopEndNodeData]):
     _node_data_cls = LoopEndNodeData
     _node_type = NodeType.LOOP_END
 
+    @classmethod
+    def version(cls) -> str:
+        return "1"
+
     def _run(self) -> NodeRunResult:
         """
         Run the node.

api/core/workflow/nodes/loop/loop_node.py

@@ -54,6 +54,10 @@ class LoopNode(BaseNode[LoopNodeData]):
     _node_data_cls = LoopNodeData
     _node_type = NodeType.LOOP
 
+    @classmethod
+    def version(cls) -> str:
+        return "1"
+
     def _run(self) -> Generator[NodeEvent | InNodeEvent, None, None]:
         """Run the node."""
         # Get inputs
@@ -482,6 +486,13 @@ class LoopNode(BaseNode[LoopNodeData]):
 
             variable_mapping.update(sub_node_variable_mapping)
 
+        for loop_variable in node_data.loop_variables or []:
+            if loop_variable.value_type == "variable":
+                assert loop_variable.value is not None, "Loop variable value must be provided for variable type"
+                # add loop variable to variable mapping
+                selector = loop_variable.value
+                variable_mapping[f"{node_id}.{loop_variable.label}"] = selector
+
         # remove variable out from loop
         variable_mapping = {
             key: value for key, value in variable_mapping.items() if value[0] not in loop_graph.node_ids

api/core/workflow/nodes/loop/loop_start_node.py

@@ -13,6 +13,10 @@ class LoopStartNode(BaseNode[LoopStartNodeData]):
     _node_data_cls = LoopStartNodeData
     _node_type = NodeType.LOOP_START
 
+    @classmethod
+    def version(cls) -> str:
+        return "1"
+
     def _run(self) -> NodeRunResult:
         """
         Run the node.