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/thread_resume.rs

@@ -74,10 +74,10 @@ async fn thread_resume_returns_rollout_history() -> Result<()> {
     create_config_toml(codex_home.path(), &server.uri())?;
 
     let preview = "Saved user message";
-    let text_elements = vec![TextElement {
-        byte_range: ByteRange { start: 0, end: 5 },
-        placeholder: Some("<note>".into()),
-    }];
+    let text_elements = vec![TextElement::new(
+        ByteRange { start: 0, end: 5 },
+        Some("<note>".into()),
+    )];
     let conversation_id = create_fake_rollout_with_text_elements(
         codex_home.path(),
         "2025-01-05T12-00-00",