Add text element metadata to protocol, app server, and core (#9331)

The second part of breaking up PR
https://github.com/openai/codex/pull/9116

Summary:

- Add `TextElement` / `ByteRange` to protocol user inputs and user
message events with defaults.
- Thread `text_elements` through app-server v1/v2 request handling and
history rebuild.
- Preserve UI metadata only in user input/events (not `ContentItem`)
while keeping local image attachments in user events for rehydration.

Details:

- Protocol: `UserInput::Text` carries `text_elements`;
`UserMessageEvent` carries `text_elements` + `local_images`.
Serialization includes empty vectors for backward compatibility.
- app-server-protocol: v1 defines `V1TextElement` / `V1ByteRange` in
camelCase with conversions; v2 uses its own camelCase wrapper.
- app-server: v1/v2 input mapping includes `text_elements`; thread
history rebuilds include them.
- Core: user event emission preserves UI metadata while model history
stays clean; history replay round-trips the metadata.

codex-rs/core/tests/suite/resume.rs

@@ -1,6 +1,8 @@
 use anyhow::Result;
 use codex_core::protocol::EventMsg;
 use codex_core::protocol::Op;
+use codex_protocol::user_input::ByteRange;
+use codex_protocol::user_input::TextElement;
 use codex_protocol::user_input::UserInput;
 use core_test_support::responses::ev_assistant_message;
 use core_test_support::responses::ev_completed;
@@ -12,6 +14,7 @@ use core_test_support::responses::start_mock_server;
 use core_test_support::skip_if_no_network;
 use core_test_support::test_codex::test_codex;
 use core_test_support::wait_for_event;
+use pretty_assertions::assert_eq;
 use std::sync::Arc;
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
@@ -32,11 +35,16 @@ async fn resume_includes_initial_messages_from_rollout_events() -> Result<()> {
     ]);
     mount_sse_once(&server, initial_sse).await;
 
+    let text_elements = vec![TextElement {
+        byte_range: ByteRange { start: 0, end: 6 },
+        placeholder: Some("<note>".into()),
+    }];
+
     codex
         .submit(Op::UserInput {
             items: vec![UserInput::Text {
                 text: "Record some messages".into(),
-                text_elements: Vec::new(),
+                text_elements: text_elements.clone(),
             }],
             final_output_json_schema: None,
         })
@@ -57,6 +65,7 @@ async fn resume_includes_initial_messages_from_rollout_events() -> Result<()> {
             EventMsg::TokenCount(_),
         ] => {
             assert_eq!(first_user.message, "Record some messages");
+            assert_eq!(first_user.text_elements, text_elements);
             assert_eq!(assistant_message.message, "Completed first turn");
         }
         other => panic!("unexpected initial messages after resume: {other:#?}"),