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/common/test_codex.rs

@@ -11,7 +11,6 @@ use codex_core::ModelProviderInfo;
 use codex_core::built_in_model_providers;
 use codex_core::config::Config;
 use codex_core::features::Feature;
-use codex_core::model_family::find_family_for_model;
 use codex_core::protocol::AskForApproval;
 use codex_core::protocol::EventMsg;
 use codex_core::protocol::Op;
@@ -71,7 +70,6 @@ impl TestCodexBuilder {
         let new_model = model.to_string();
         self.with_config(move |config| {
             config.model = new_model.clone();
-            config.model_family = find_family_for_model(&new_model).expect("model family");
         })
     }
 

codex-rs/core/tests/suite/client.rs

@@ -16,7 +16,7 @@ use codex_core::auth::AuthCredentialsStoreMode;
 use codex_core::built_in_model_providers;
 use codex_core::error::CodexErr;
 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::EventMsg;
 use codex_core::protocol::Op;
 use codex_core::protocol::SessionSource;
@@ -1378,8 +1378,7 @@ async fn context_window_error_sets_total_tokens_to_model_window() -> anyhow::Res
     let TestCodex { codex, .. } = test_codex()
         .with_config(|config| {
             config.model = "gpt-5.1".to_string();
-            config.model_family =
-                find_family_for_model("gpt-5.1").expect("known gpt-5.1 model family");
+            config.model_family = find_family_for_model("gpt-5.1");
             config.model_context_window = Some(272_000);
         })
         .build(&server)

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?;

codex-rs/core/tests/suite/shell_serialization.rs

@@ -4,7 +4,7 @@
 use anyhow::Result;
 use codex_core::config::Config;
 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::SandboxPolicy;
 use core_test_support::assert_regex_match;
 use core_test_support::responses::ev_assistant_message;
@@ -46,13 +46,12 @@ fn configure_shell_command_model(output_type: ShellModelOutput, config: &mut Con
         return;
     }
 
-    if let Some(shell_command_family) = find_family_for_model("test-gpt-5-codex") {
-        if config.model_family.shell_type == shell_command_family.shell_type {
-            return;
-        }
-        config.model = shell_command_family.slug.clone();
-        config.model_family = shell_command_family;
+    let shell_command_family = find_family_for_model("test-gpt-5-codex");
+    if config.model_family.shell_type == shell_command_family.shell_type {
+        return;
     }
+    config.model = shell_command_family.slug.clone();
+    config.model_family = shell_command_family;
 }
 
 fn shell_responses(