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/tests/suite/model_tools.rs

@@ -7,8 +7,8 @@ use codex_core::built_in_model_providers;
 use codex_core::features::Feature;
 use codex_core::model_family::find_family_for_model;
 use codex_core::protocol::EventMsg;
-use codex_core::protocol::InputItem;
 use codex_core::protocol::Op;
+use codex_protocol::user_input::UserInput;
 use core_test_support::load_default_config_for_test;
 use core_test_support::load_sse_fixture_with_id;
 use core_test_support::responses;
@@ -74,7 +74,7 @@ async fn collect_tool_identifiers_for_model(model: &str) -> Vec<String> {
 
     codex
         .submit(Op::UserInput {
-            items: vec![InputItem::Text {
+            items: vec![UserInput::Text {
                 text: "hello tools".into(),
             }],
         })