Add ItemStarted/ItemCompleted events for UserInputItem (#5306)

Adds a new ItemStarted event and delivers UserMessage as the first item
type (more to come).


Renames `InputItem` to `UserInput` considering we're using the `Item`
suffix for actual items.

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

@@ -5,8 +5,8 @@ use tokio_util::sync::CancellationToken;
 
 use crate::codex::TurnContext;
 use crate::codex::run_task;
-use crate::protocol::InputItem;
 use crate::state::TaskKind;
+use codex_protocol::user_input::UserInput;
 
 use super::SessionTask;
 use super::SessionTaskContext;
@@ -25,7 +25,7 @@ impl SessionTask for RegularTask {
         session: Arc<SessionTaskContext>,
         ctx: Arc<TurnContext>,
         sub_id: String,
-        input: Vec<InputItem>,
+        input: Vec<UserInput>,
         cancellation_token: CancellationToken,
     ) -> Option<String> {
         let sess = session.clone_session();