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/tests/suite/prompt_caching.rs

@@ -1,7 +1,7 @@
 #![allow(clippy::unwrap_used)]
 
 use codex_core::features::Feature;
-use codex_core::model_family::find_family_for_model;
+use codex_core::openai_models::model_family::find_family_for_model;
 use codex_core::protocol::AskForApproval;
 use codex_core::protocol::ENVIRONMENT_CONTEXT_OPEN_TAG;
 use codex_core::protocol::EventMsg;
@@ -74,7 +74,6 @@ async fn codex_mini_latest_tools() -> anyhow::Result<()> {
             config.features.disable(Feature::ApplyPatchFreeform);
             config.model = "codex-mini-latest".to_string();
             config.model_family = find_family_for_model("codex-mini-latest")
-                .expect("model family for codex-mini-latest");
         })
         .build(&server)
         .await?;