Migrate model family to models manager (#7565)

This PR moves `ModelsFamily` to `openai_models`. It also propagates
`ModelsManager` to session services and use it to drive model family. We
also make `derive_default_model_family` private because it's a step
towards what we want: one place that gives model configuration.

This is a second step at having one source of truth for models
information and config: `ModelsManager`.

Next steps would be to remove `ModelsFamily` from config. That's massive
because it's being used in 41 occasions mostly pre launching `codex`.
Also, we need to make `find_family_for_model` private. It's also big
because it's being used in 21 occasions ~ all tests.

codex-rs/core/src/lib.rs

@@ -67,7 +67,6 @@ pub use conversation_manager::NewConversation;
 pub use auth::AuthManager;
 pub use auth::CodexAuth;
 pub mod default_client;
-pub mod model_family;
 mod openai_model_info;
 pub mod project_doc;
 mod rollout;