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/core/tests/suite/skills.rs

@@ -64,6 +64,7 @@ async fn user_turn_includes_skill_instructions() -> Result<()> {
             items: vec![
                 UserInput::Text {
                     text: "please use $demo".to_string(),
+                    text_elements: Vec::new(),
                 },
                 UserInput::Skill {
                     name: "demo".to_string(),