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/request_compression.rs

@@ -39,6 +39,7 @@ async fn request_body_is_zstd_compressed_for_codex_backend_when_enabled() -> any
         .submit(Op::UserInput {
             items: vec![UserInput::Text {
                 text: "compress me".into(),
+                text_elements: Vec::new(),
             }],
             final_output_json_schema: None,
         })
@@ -82,6 +83,7 @@ async fn request_body_is_not_compressed_for_api_key_auth_even_when_enabled() ->
         .submit(Op::UserInput {
             items: vec![UserInput::Text {
                 text: "do not compress".into(),
+                text_elements: Vec::new(),
             }],
             final_output_json_schema: None,
         })