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/mod.rs

@@ -17,13 +17,13 @@ use crate::codex::Session;
 use crate::codex::TurnContext;
 use crate::protocol::Event;
 use crate::protocol::EventMsg;
-use crate::protocol::InputItem;
 use crate::protocol::TaskCompleteEvent;
 use crate::protocol::TurnAbortReason;
 use crate::protocol::TurnAbortedEvent;
 use crate::state::ActiveTurn;
 use crate::state::RunningTask;
 use crate::state::TaskKind;
+use codex_protocol::user_input::UserInput;
 
 pub(crate) use compact::CompactTask;
 pub(crate) use regular::RegularTask;
@@ -56,7 +56,7 @@ pub(crate) trait SessionTask: Send + Sync + 'static {
         session: Arc<SessionTaskContext>,
         ctx: Arc<TurnContext>,
         sub_id: String,
-        input: Vec<InputItem>,
+        input: Vec<UserInput>,
         cancellation_token: CancellationToken,
     ) -> Option<String>;
 
@@ -70,7 +70,7 @@ impl Session {
         self: &Arc<Self>,
         turn_context: Arc<TurnContext>,
         sub_id: String,
-        input: Vec<InputItem>,
+        input: Vec<UserInput>,
         task: T,
     ) {
         self.abort_all_tasks(TurnAbortReason::Replaced).await;