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

     _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

api/core/workflow/nodes/knowledge_retrieval/knowledge_retrieval_node.py

     _node_data_cls = KnowledgeRetrievalNodeData  # type: ignore
     _node_type = NodeType.KNOWLEDGE_RETRIEVAL
 
+    @classmethod
+    def version(cls):
+        return "1"
+
     def _run(self) -> NodeRunResult:  # type: ignore
         node_data = cast(KnowledgeRetrievalNodeData, self.node_data)
         # extract variables

api/core/workflow/nodes/question_classifier/question_classifier_node.py

     _node_data_cls = QuestionClassifierNodeData  # type: ignore
     _node_type = NodeType.QUESTION_CLASSIFIER
 
+    @classmethod
+    def version(cls):
+        return "1"
+
     def _run(self):
         node_data = cast(QuestionClassifierNodeData, self.node_data)
         variable_pool = self.graph_runtime_state.variable_pool

api/tests/unit_tests/core/workflow/nodes/base/test_base_node.py

+from core.workflow.nodes.base.node import BaseNode
+from core.workflow.nodes.enums import NodeType
+
+# Ensures that all node classes are imported.
+from core.workflow.nodes.node_mapping import NODE_TYPE_CLASSES_MAPPING
+
+_ = NODE_TYPE_CLASSES_MAPPING
+
+
+def _get_all_subclasses(root: type[BaseNode]) -> list[type[BaseNode]]:
+    subclasses = []
+    queue = [root]
+    while queue:
+        cls = queue.pop()
+
+        subclasses.extend(cls.__subclasses__())
+        queue.extend(cls.__subclasses__())
+
+    return subclasses
+
+
+def test_ensure_subclasses_of_base_node_has_node_type_and_version_method_defined():
+    classes = _get_all_subclasses(BaseNode)  # type: ignore
+    type_version_set: set[tuple[NodeType, str]] = set()
+
+    for cls in classes:
+        # Validate that 'version' is directly defined in the class (not inherited) by checking the class's __dict__
+        assert "version" in cls.__dict__, f"class {cls} should have version method defined (NOT INHERITED.)"
+        node_type = cls._node_type
+        node_version = cls.version()
+
+        assert isinstance(cls._node_type, NodeType)
+        assert isinstance(node_version, str)
+        node_type_and_version = (node_type, node_version)
+        assert node_type_and_version not in type_version_set
+        type_version_set.add(node_type_and_version)