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/app-server/tests/suite/v2/thread_resume.rs

@@ -1,6 +1,6 @@
 use anyhow::Result;
 use app_test_support::McpProcess;
-use app_test_support::create_fake_rollout;
+use app_test_support::create_fake_rollout_with_text_elements;
 use app_test_support::create_mock_responses_server_repeating_assistant;
 use app_test_support::to_response;
 use codex_app_server_protocol::JSONRPCResponse;
@@ -15,6 +15,9 @@ use codex_app_server_protocol::TurnStatus;
 use codex_app_server_protocol::UserInput;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseItem;
+use codex_protocol::user_input::ByteRange;
+use codex_protocol::user_input::TextElement;
+use pretty_assertions::assert_eq;
 use std::path::PathBuf;
 use tempfile::TempDir;
 use tokio::time::timeout;
@@ -71,11 +74,19 @@ async fn thread_resume_returns_rollout_history() -> Result<()> {
     create_config_toml(codex_home.path(), &server.uri())?;
 
     let preview = "Saved user message";
-    let conversation_id = create_fake_rollout(
+    let text_elements = vec![TextElement {
+        byte_range: ByteRange { start: 0, end: 5 },
+        placeholder: Some("<note>".into()),
+    }];
+    let conversation_id = create_fake_rollout_with_text_elements(
         codex_home.path(),
         "2025-01-05T12-00-00",
         "2025-01-05T12:00:00Z",
         preview,
+        text_elements
+            .iter()
+            .map(|elem| serde_json::to_value(elem).expect("serialize text element"))
+            .collect(),
         Some("mock_provider"),
         None,
     )?;
@@ -119,7 +130,7 @@ async fn thread_resume_returns_rollout_history() -> Result<()> {
                 content,
                 &vec![UserInput::Text {
                     text: preview.to_string(),
-                    text_elements: Vec::new(),
+                    text_elements: text_elements.clone().into_iter().map(Into::into).collect(),
                 }]
             );
         }