bug: APISIX 3.17.0 Built-in Dashboard: Deleting one plugin in Plugin Config will remove all plugins

### Current Behavior

Current Behavior
In the built-in Dashboard of APISIX 3.17.0, when editing a Plugin Config that contains multiple plugins:

- Click the **Delete** button on any single plugin card

- Click **Submit** to save

- **All plugins are cleared** (plugins: {} in the request payload)

This is a **frontend UI bug**, not an APISIX core bug.

### Expected Behavior

- Deleting one plugin should only remove that plugin

- All other plugins remain unchanged

- Submit and save correctly

### Error Logs

_No response_

### Steps to Reproduce

1.Open APISIX 3.17.0 built-in Dashboard
2.Go to Plugin Config
3.Create or edit a Plugin Config with two or more plugins (e.g., cors, prometheus, limit-req)
4.Delete one plugin using the delete icon on the plugin card
5.Click Submit
6.Re-enter edit page: all plugins are gone

### Environment

- APISIX version: 3.17.0
- Dashboard: Built-in
- Browser: Chrome / Edge / Firefox (all reproduce)
- Deployment: Binary / Docker (both reproduce)
- OS: Linux