Pass TurnContext around instead of sub_id (#5421)

Today `sub_id` is an ID of a single incoming Codex Op submition. We then
associate all events triggered by this operation using the same
`sub_id`.

At the same time we are also creating a TurnContext per submission and
we'd like to start associating some events (item added/item completed)
with an entire turn instead of just the operation that started it.

Using turn context when sending events give us flexibility to change
notification scheme.

codex-rs/core/src/tasks/regular.rs

@@ -24,19 +24,10 @@ impl SessionTask for RegularTask {
         self: Arc<Self>,
         session: Arc<SessionTaskContext>,
         ctx: Arc<TurnContext>,
-        sub_id: String,
         input: Vec<UserInput>,
         cancellation_token: CancellationToken,
     ) -> Option<String> {
         let sess = session.clone_session();
-        run_task(
-            sess,
-            ctx,
-            sub_id,
-            input,
-            TaskKind::Regular,
-            cancellation_token,
-        )
-        .await
+        run_task(sess, ctx, input, TaskKind::Regular, cancellation_token).await
     }
 }