[Bug] Backend retains stale modelSlug after inserting BYOK

[4ndrelim]

Describe the bug
After inserting BYOK, if the user did not change the model and tried to continue the conversation, the user may face the following error. This is because openai/ prefix exists in the modelSlug that was originally for OpenRouter's. The backend uses the stale modelSlug that is not updated to use OpenAI's one without the prefix.

To Reproduce
Steps to reproduce the behavior:

1. Initiate a conversation as per normal (without BYOK)
2. Provide your own API key
3. Try to continue conversation
4. See error

Expected behavior
Error should not happen. ModelSlug needs to be updated.

Screenshots