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/services/workflow_draft_variable_service.py

@@ -129,7 +129,8 @@ class WorkflowDraftVariableService:
     ) -> list[WorkflowDraftVariable]:
         ors = []
         for selector in selectors:
-            node_id, name = selector
+            assert len(selector) >= MIN_SELECTORS_LENGTH, f"Invalid selector to get: {selector}"
+            node_id, name = selector[:2]
             ors.append(and_(WorkflowDraftVariable.node_id == node_id, WorkflowDraftVariable.name == name))
 
         # NOTE(QuantumGhost): Although the number of `or` expressions may be large, as long as