Add text element metadata to types (#9235)

Initial type tweaking PR to make the diff of
https://github.com/openai/codex/pull/9116 smaller

This should not change any behavior, just adds some fields to types

codex-rs/app-server/tests/suite/v2/thread_fork.rs

@@ -95,7 +95,8 @@ async fn thread_fork_creates_new_thread_and_emits_started() -> Result<()> {
             assert_eq!(
                 content,
                 &vec![UserInput::Text {
-                    text: preview.to_string()
+                    text: preview.to_string(),
+                    text_elements: Vec::new(),
                 }]
             );
         }