Persist text elements through TUI input and history (#9393)

Continuation of breaking up this PR
https://github.com/openai/codex/pull/9116

## Summary
- Thread user text element ranges through TUI/TUI2 input, submission,
queueing, and history so placeholders survive resume/edit flows.
- Preserve local image attachments alongside text elements and rehydrate
placeholders when restoring drafts.
- Keep model-facing content shapes clean by attaching UI metadata only
to user input/events (no API content changes).

## Key Changes
- TUI/TUI2 composer now captures text element ranges, trims them with
text edits, and restores them when submission is suppressed.
- User history cells render styled spans for text elements and keep
local image paths for future rehydration.
- Initial chat widget bootstraps accept empty `initial_text_elements` to
keep initialization uniform.
- Protocol/core helpers updated to tolerate the new InputText field
shape without changing payloads sent to the API.

codex-rs/tui/src/app_backtrack.rs

@@ -204,7 +204,10 @@ impl App {
         });
         self.chat_widget.submit_op(Op::ThreadRollback { num_turns });
         if !prefill.is_empty() {
-            self.chat_widget.set_composer_text(prefill);
+            // TODO: Rehydrate text_elements/local_image_paths from the selected user cell so
+            // backtrack preserves image placeholders and attachments.
+            self.chat_widget
+                .set_composer_text(prefill, Vec::new(), Vec::new());
         }
     }
 
@@ -554,6 +557,8 @@ mod tests {
         let mut cells: Vec<Arc<dyn HistoryCell>> = vec![
             Arc::new(UserHistoryCell {
                 message: "first user".to_string(),
+                text_elements: Vec::new(),
+                local_image_paths: Vec::new(),
             }) as Arc<dyn HistoryCell>,
             Arc::new(AgentMessageCell::new(vec![Line::from("assistant")], true))
                 as Arc<dyn HistoryCell>,
@@ -570,6 +575,8 @@ mod tests {
                 as Arc<dyn HistoryCell>,
             Arc::new(UserHistoryCell {
                 message: "first".to_string(),
+                text_elements: Vec::new(),
+                local_image_paths: Vec::new(),
             }) as Arc<dyn HistoryCell>,
             Arc::new(AgentMessageCell::new(vec![Line::from("after")], false))
                 as Arc<dyn HistoryCell>,
@@ -598,11 +605,15 @@ mod tests {
                 as Arc<dyn HistoryCell>,
             Arc::new(UserHistoryCell {
                 message: "first".to_string(),
+                text_elements: Vec::new(),
+                local_image_paths: Vec::new(),
             }) as Arc<dyn HistoryCell>,
             Arc::new(AgentMessageCell::new(vec![Line::from("between")], false))
                 as Arc<dyn HistoryCell>,
             Arc::new(UserHistoryCell {
                 message: "second".to_string(),
+                text_elements: Vec::new(),
+                local_image_paths: Vec::new(),
             }) as Arc<dyn HistoryCell>,
             Arc::new(AgentMessageCell::new(vec![Line::from("tail")], false))
                 as Arc<dyn HistoryCell>,