feat(ui) - swap OpenAI API with LF API

[andrewrisse]

User Story

As a frontend developer
I want to use the OpenAI API instead of Leapfrog API
So that I can build new features for the UI before the backend capacility is ready. I want to use real OpenAI so that we can also test Leapfrog API's conformity with the OpenAI spec.

Acceptance Criteria

Given I change my environment variables for the frontend application
When I use the app
Then I am getting responses from OpenAI and I can use assistants with OpenAI

Describe alternatives you've considered

We could just mock these endpoints, but if we use real OpenAI, we can also test Leapfrog API's conformity with the OpenAI spec.

Additional context

Will need to update naming conventions to use threads instead of conversations. Also try to use more types from Vercel AI SDK instead of our own types.