bug: OpenAI models now a part of its extension

[louis-jan]

Describe the bug OpenAI models should not be pre-populated via by model extension like other, OpenAI extension will provision models and should be enabled by default.

Action Items:

• [ ] Disable OpenAI extension by default
• [ ] Models will only be populated after user enable the extension
• [ ] Proper error message, see #995

Expected behavior

• Fresh install Jan
• Onboarding screen should lead me to download models
• I can go to settings and enable OpenAI
• I can engine settings there
• I can create thread with OpenAI model then