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/v2.rs

@@ -1665,24 +1665,38 @@ pub struct TextElement {
     /// Byte range in the parent `text` buffer that this element occupies.
     pub byte_range: ByteRange,
     /// Optional human-readable placeholder for the element, displayed in the UI.
-    pub placeholder: Option<String>,
+    placeholder: Option<String>,
+}
+
+impl TextElement {
+    pub fn new(byte_range: ByteRange, placeholder: Option<String>) -> Self {
+        Self {
+            byte_range,
+            placeholder,
+        }
+    }
+
+    pub fn set_placeholder(&mut self, placeholder: Option<String>) {
+        self.placeholder = placeholder;
+    }
+
+    pub fn placeholder(&self) -> Option<&str> {
+        self.placeholder.as_deref()
+    }
 }
 
 impl From<CoreTextElement> for TextElement {
     fn from(value: CoreTextElement) -> Self {
-        Self {
-            byte_range: value.byte_range.into(),
-            placeholder: value.placeholder,
-        }
+        Self::new(
+            value.byte_range.into(),
+            value._placeholder_for_conversion_only().map(str::to_string),
+        )
     }
 }
 
 impl From<TextElement> for CoreTextElement {
     fn from(value: TextElement) -> Self {
-        Self {
-            byte_range: value.byte_range.into(),
-            placeholder: value.placeholder,
-        }
+        Self::new(value.byte_range.into(), value.placeholder)
     }
 }