feat(api): Explicitly define version method for all BaseNode subclasses (#21443)

This PR addresses issue #21441 by implementing explicit `version` method definitions for all `BaseNode` subclasses to improve code maintainability.

### Changes

Added explicit `version` method definitions for all `BaseNode` subclasses:

- `QuestionClassifierNode`
- `KnowledgeRetrievalNode` 
- `AgentNode`

Added comprehensive test suite to validate:

1. All subclasses of `BaseNode` have explicitly defined `version` method
2. All subclasses have required `_node_type` property
3. The `(node_type, node_version)` combination is unique across all subclasses

api/core/workflow/nodes/agent/agent_node.py

@@ -39,6 +39,10 @@ class AgentNode(ToolNode):
     _node_data_cls = AgentNodeData  # type: ignore
     _node_type = NodeType.AGENT
 
+    @classmethod
+    def version(cls) -> str:
+        return "1"
+
     def _run(self) -> Generator:
         """
         Run the agent node