ref: state - 2 (#4229)

Extracting tasks in a module and start abstraction behind a Trait (more
to come on this but each task will be tackled in a dedicated PR)
The goal was to drop the ActiveTask and to have a (potentially) set of
tasks during each turn

codex-rs/core/src/lib.rs

@@ -77,6 +77,7 @@ pub use rollout::list::ConversationsPage;
 pub use rollout::list::Cursor;
 mod function_tool;
 mod state;
+mod tasks;
 mod user_notification;
 pub mod util;