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/mod.rs

@@ -11,6 +11,7 @@ mod compact_resume_fork;
 mod exec;
 mod fork_conversation;
 mod grep_files;
+mod items;
 mod json_result;
 mod list_dir;
 mod live_cli;