chore: change built_in_model_providers so "openai" is the only "bundled" provider

[bolinfest]

As we are close to releasing the Rust CLI beta, for the moment, let's take a more neutral stance on what it takes to be a "built-in" provider.

• For example, there seems to be a discrepancy around what the "right" configuration for Gemini is: fix: update Gemini base URL to the correct endpoint #881
• And while the current list of "built-in" providers are all arguably "well-known" names, this raises a question of what to do about potentially less familiar providers, such as Add Portkey-AI Provider #1142. Do we just accept every pull request like this, or is there some criteria a provider has to meet to "qualify" to be bundled with Codex CLI?

I think that if we can establish clear ground rules for being a built-in provider, then we can bring this back. But until then, I would rather take a minimalist approach because if we decided to reverse our position later, it would break folks who were depending on the presence of the built-in providers.