chore: introduce ModelFamily abstraction (#1838)

To date, we have a number of hardcoded OpenAI model slug checks spread
throughout the codebase, which makes it hard to audit the various
special cases for each model. To mitigate this issue, this PR introduces
the idea of a `ModelFamily` that has fields to represent the existing
special cases, such as `supports_reasoning_summaries` and
`uses_local_shell_tool`.

There is a `find_family_for_model()` function that maps the raw model
slug to a `ModelFamily`. This function hardcodes all the knowledge about
the special attributes for each model. This PR then replaces the
hardcoded model name checks with checks against a `ModelFamily`.

Note `ModelFamily` is now available as `Config::model_family`. We should
ultimately remove `Config::model` in favor of
`Config::model_family::slug`.

codex-rs/core/src/lib.rs

@@ -31,6 +31,7 @@ mod model_provider_info;
 pub use model_provider_info::ModelProviderInfo;
 pub use model_provider_info::WireApi;
 pub use model_provider_info::built_in_model_providers;
+pub mod model_family;
 mod models;
 mod openai_model_info;
 mod openai_tools;
@@ -47,5 +48,4 @@ mod user_notification;
 pub mod util;
 
 pub use apply_patch::CODEX_APPLY_PATCH_ARG1;
-pub use client_common::model_supports_reasoning_summaries;
 pub use safety::get_platform_sandbox;