Add total (non-partial) TextElement placeholder accessors (#9545)

## Summary
- Make `TextElement` placeholders private and add a text-backed accessor
to avoid assuming `Some`.
- Since they are optional in the protocol, we want to make sure any
accessors properly handle the None case (getting the placeholder using
the byte range in the text)
- Preserve placeholders during protocol/app-server conversions using the
accessor fallback.
- Update TUI composer/remap logic and tests to use the new
constructor/accessor.

codex-rs/app-server/tests/suite/v2/turn_start.rs

@@ -142,10 +142,10 @@ async fn turn_start_emits_user_message_item_with_text_elements() -> Result<()> {
     .await??;
     let ThreadStartResponse { thread, .. } = to_response::<ThreadStartResponse>(thread_resp)?;
 
-    let text_elements = vec![TextElement {
-        byte_range: ByteRange { start: 0, end: 5 },
-        placeholder: Some("<note>".to_string()),
-    }];
+    let text_elements = vec![TextElement::new(
+        ByteRange { start: 0, end: 5 },
+        Some("<note>".to_string()),
+    )];
     let turn_req = mcp
         .send_turn_start_request(TurnStartParams {
             thread_id: thread.id.clone(),