feat(api): Introduce WorkflowDraftVariable Model (#19737)

- Introduce `WorkflowDraftVariable` model and the corresponding migration.
- Implement `EnumText`,  a custom column type for SQLAlchemy designed
  to work seamlessly with enumeration classes based on `StrEnum`.

api/core/repositories/sqlalchemy_workflow_node_execution_repository.py

@@ -4,13 +4,14 @@ SQLAlchemy implementation of the WorkflowNodeExecutionRepository.
 
 import json
 import logging
-from collections.abc import Sequence
-from typing import Optional, Union
+from collections.abc import Mapping, Sequence
+from typing import Any, Optional, Union, cast
 
 from sqlalchemy import UnaryExpression, asc, delete, desc, select
 from sqlalchemy.engine import Engine
 from sqlalchemy.orm import sessionmaker
 
+from core.workflow.entities.node_entities import NodeRunMetadataKey
 from core.workflow.entities.node_execution_entities import (
     NodeExecution,
     NodeExecutionStatus,
@@ -122,7 +123,12 @@ class SQLAlchemyWorkflowNodeExecutionRepository(WorkflowNodeExecutionRepository)
             status=status,
             error=db_model.error,
             elapsed_time=db_model.elapsed_time,
-            metadata=metadata,
+            # FIXME(QuantumGhost): a temporary workaround for the following type check failure in Python 3.11.
+            # However, this problem is not occurred in Python 3.12.
+            #
+            # A case of this error is:
+            # https://github.com/langgenius/dify/actions/runs/15112698604/job/42475659482?pr=19737#step:9:24
+            metadata=cast(Mapping[NodeRunMetadataKey, Any] | None, metadata),
             created_at=db_model.created_at,
             finished_at=db_model.finished_at,
         )

api/core/variables/consts.py

@@ -0,0 +1,7 @@
+# The minimal selector length for valid variables.
+#
+# The first element of the selector is the node id, and the second element is the variable name.
+#
+# If the selector length is more than 2, the remaining parts are the keys / indexes paths used
+# to extract part of the variable value.
+MIN_SELECTORS_LENGTH = 2

api/core/variables/utils.py

@@ -0,0 +1,8 @@
+from collections.abc import Iterable, Sequence
+
+
+def to_selector(node_id: str, name: str, paths: Iterable[str] = ()) -> Sequence[str]:
+    selectors = [node_id, name]
+    if paths:
+        selectors.extend(paths)
+    return selectors