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/tasks/regular.rs

@@ -0,0 +1,32 @@
+use std::sync::Arc;
+
+use async_trait::async_trait;
+
+use crate::codex::TurnContext;
+use crate::codex::run_task;
+use crate::protocol::InputItem;
+use crate::state::TaskKind;
+
+use super::SessionTask;
+use super::SessionTaskContext;
+
+#[derive(Clone, Copy, Default)]
+pub(crate) struct RegularTask;
+
+#[async_trait]
+impl SessionTask for RegularTask {
+    fn kind(&self) -> TaskKind {
+        TaskKind::Regular
+    }
+
+    async fn run(
+        self: Arc<Self>,
+        session: Arc<SessionTaskContext>,
+        ctx: Arc<TurnContext>,
+        sub_id: String,
+        input: Vec<InputItem>,
+    ) -> Option<String> {
+        let sess = session.clone_session();
+        run_task(sess, ctx, sub_id, input).await
+    }
+}