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/send_message.rs

@@ -101,6 +101,7 @@ async fn send_message(
             conversation_id,
             items: vec![InputItem::Text {
                 text: message.to_string(),
+                text_elements: Vec::new(),
             }],
         })
         .await?;
@@ -194,6 +195,7 @@ async fn test_send_message_raw_notifications_opt_in() -> Result<()> {
             conversation_id,
             items: vec![InputItem::Text {
                 text: "Hello".to_string(),
+                text_elements: Vec::new(),
             }],
         })
         .await?;
@@ -245,6 +247,7 @@ async fn test_send_message_session_not_found() -> Result<()> {
             conversation_id: unknown,
             items: vec![InputItem::Text {
                 text: "ping".to_string(),
+                text_elements: Vec::new(),
             }],
         })
         .await?;
@@ -425,7 +428,7 @@ fn content_texts(content: &[ContentItem]) -> Vec<&str> {
     content
         .iter()
         .filter_map(|item| match item {
-            ContentItem::InputText { text } | ContentItem::OutputText { text } => {
+            ContentItem::InputText { text, .. } | ContentItem::OutputText { text } => {
                 Some(text.as_str())
             }
             _ => None,