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/state/mod.rs

@@ -5,4 +5,5 @@ mod turn;
 pub(crate) use service::SessionServices;
 pub(crate) use session::SessionState;
 pub(crate) use turn::ActiveTurn;
-pub(crate) use turn::TurnState;
+pub(crate) use turn::RunningTask;
+pub(crate) use turn::TaskKind;