The API seems to return nested errors, so made the error deserialise
properly in case we get `{ "error": {...} }` instead of a top-level
error

Closes #57024

For testing, you can prompt something like: `tell me about
https://registry.npmjs.org/vite-plus.`

Before:

<img width="631" height="69" alt="image"
src="https://github.com/user-attachments/assets/5d02e7ec-8176-4bff-87d7-908ac8f0b498"
/>

After:

<img width="697" height="61" alt="image"
src="https://github.com/user-attachments/assets/97fac249-8b76-463c-8483-a150f5db9857"
/>


Release Notes:

- openai: Fixed an issue where error messages would not show up properly