Feat: Keep connection status during generating agent by drag and drop … (#10141)

### What problem does this PR solve?

About issue #10140

In version 0.20.1, we implemented the generation of new node through
mouse drag and drop. If we could create a workflow module like in Coze,
where there is not only a dropdown menu but also an intermediate node
(placeholder node) after the drag and drop is completed, this could
improve the user experience.

### Type of change

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

web/src/pages/agent/hooks/use-add-node.ts

@@ -336,6 +336,7 @@ export function useAddNode(reactFlowInstance?: ReactFlowInstance<any, any>) {
             x: 0,
             y: 0,
           },
+          draggable: type === Operator.Placeholder ? false : undefined,
           data: {
             label: `${type}`,
             name: generateNodeNamesWithIncreasingIndex(