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-protocol/src/protocol/v1.rs

@@ -503,17 +503,14 @@ impl From<CoreTextElement> for V1TextElement {
     fn from(value: CoreTextElement) -> Self {
         Self {
             byte_range: value.byte_range.into(),
-            placeholder: value.placeholder,
+            placeholder: value._placeholder_for_conversion_only().map(str::to_string),
         }
     }
 }
 
 impl From<V1TextElement> for CoreTextElement {
     fn from(value: V1TextElement) -> Self {
-        Self {
-            byte_range: value.byte_range.into(),
-            placeholder: value.placeholder,
-        }
+        Self::new(value.byte_range.into(), value.placeholder)
     }
 }