[V0.5] When the /api/workflow/generate interface returns an error, the response code is 20000 (expected: 50000).

[wanghuan-520]

Description:
When the /api/workflow/generate interface returns an error, the response code is 20000 (expected: 50000).

Steps to Reproduce:
1.On the workflow page, click the "New Workflow" button.
2.In the "Generate Workflow with AI" pop-up, enter a prompt with fewer than 15 characters.

CURL

Current Behavior:
The interface returns an error with code 20000, and no toast notification is displayed on the frontend.

Expected Behavior:
The interface should return an error code 50000, and the frontend should display a toast notification to alert the user.