fix cursor file name insert (#1896)

Cursor wasn't moving when inserting a file, resulting in being not at
the end of the filename when inserting the file.
This fixes it by moving the cursor to the end of the file + one trailing
space.


Example screenshot after selecting a file when typing `@`
<img width="823" height="268" alt="image"
src="https://github.com/user-attachments/assets/ec6e3741-e1ba-4752-89d2-11f14a2bd69f"
/>

codex-rs/tui/src/bottom_pane/chat_composer.rs

@@ -453,6 +453,8 @@ impl ChatComposer {
         new_text.push_str(&text[end_idx..]);
 
         self.textarea.set_text(&new_text);
+        let new_cursor = start_idx.saturating_add(path.len()).saturating_add(1);
+        self.textarea.set_cursor(new_cursor);
     }
 
     /// Handle key event when no popup is visible.