fix(api): adding variable to variable pool recursively while loading draft variables. (#21478)

This PR fix the issue that `ObjectSegment` are not recursively added to the draft variable pool while loading draft variables from database. It also fixes an issue about loading variables with more than two elements in the its selector.

Enhances #19735.
Closes #21477.

api/core/workflow/utils/variable_utils.py

@@ -0,0 +1,28 @@
+from core.variables.segments import ObjectSegment, Segment
+from core.workflow.entities.variable_pool import VariablePool, VariableValue
+
+
+def append_variables_recursively(
+    pool: VariablePool, node_id: str, variable_key_list: list[str], variable_value: VariableValue | Segment
+):
+    """
+    Append variables recursively
+    :param node_id: node id
+    :param variable_key_list: variable key list
+    :param variable_value: variable value
+    :return:
+    """
+    pool.add([node_id] + variable_key_list, variable_value)
+
+    # if variable_value is a dict, then recursively append variables
+    if isinstance(variable_value, ObjectSegment):
+        variable_dict = variable_value.value
+    elif isinstance(variable_value, dict):
+        variable_dict = variable_value
+    else:
+        return
+
+    for key, value in variable_dict.items():
+        # construct new key list
+        new_key_list = variable_key_list + [key]
+        append_variables_recursively(pool, node_id=node_id, variable_key_list=new_key_list, variable_value=value)