feat: restrict classification operators cannot be connected to Answer and other classification #918 (#1294)

### What problem does this PR solve?

feat: restrict classification operators cannot be connected to Answer
and other classification #918

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

web/src/pages/flow/canvas/node/begin-node.tsx

@@ -0,0 +1,39 @@
+import { Flex, Space } from 'antd';
+import classNames from 'classnames';
+import { Handle, NodeProps, Position } from 'reactflow';
+import { Operator } from '../../constant';
+import { NodeData } from '../../interface';
+import OperatorIcon from '../../operator-icon';
+import NodeDropdown from './dropdown';
+
+import styles from './index.less';
+
+// TODO: do not allow other nodes to connect to this node
+export function BeginNode({ id, data, selected }: NodeProps<NodeData>) {
+  return (
+    <section
+      className={classNames(styles.ragNode, {
+        [styles.selectedNode]: selected,
+      })}
+    >
+      <Handle
+        type="source"
+        position={Position.Right}
+        isConnectable
+        className={styles.handle}
+      ></Handle>
+      <Flex vertical align="center" justify="center">
+        <Space size={6}>
+          <OperatorIcon
+            name={data.label as Operator}
+            fontSize={16}
+          ></OperatorIcon>
+          <NodeDropdown id={id}></NodeDropdown>
+        </Space>
+      </Flex>
+      <section className={styles.bottomBox}>
+        <div className={styles.nodeName}>{id}</div>
+      </section>
+    </section>
+  );
+}