Prompt Expansion: Preserve Text Elements (#9518)

Summary
- Preserve `text_elements` through custom prompt argument parsing and
expansion (named and numeric placeholders).
- Translate text element ranges through Shlex parsing using sentinel
substitution, and rehydrate text + element ranges per arg.
- Drop image attachments when their placeholder does not survive prompt
expansion, keeping attachments consistent with rendered elements.
- Mirror changes in TUI2 and expand tests for prompt parsing/expansion
edge cases.

Tests
- placeholders with spaces as single tokens (positional + key=value,
quoted + unquoted),
  - prompt expansion with image placeholders,
  - large paste + image arg combinations,
  - unused image arg dropped after expansion.

codex-rs/protocol/src/user_input.rs

@@ -47,14 +47,30 @@ impl TextElement {
         }
     }
 
+    /// Returns a copy of this element with a remapped byte range.
+    ///
+    /// The placeholder is preserved as-is; callers must ensure the new range
+    /// still refers to the same logical element (and same placeholder)
+    /// within the new text.
+    pub fn map_range<F>(&self, map: F) -> Self
+    where
+        F: FnOnce(ByteRange) -> ByteRange,
+    {
+        Self {
+            byte_range: map(self.byte_range),
+            placeholder: self.placeholder.clone(),
+        }
+    }
+
     pub fn set_placeholder(&mut self, placeholder: Option<String>) {
         self.placeholder = placeholder;
     }
 
     /// Returns the stored placeholder without falling back to the text buffer.
     ///
-    /// This is intended only for protocol conversions where the full text is not
-    /// available; prefer `placeholder(text)` for UI logic.
+    /// This must only be used inside `From<TextElement>` implementations on equivalent
+    /// protocol types where the source text is unavailable. Prefer `placeholder(text)`
+    /// everywhere else.
     #[doc(hidden)]
     pub fn _placeholder_for_conversion_only(&self) -> Option<&str> {
         self.placeholder.as_deref()