refactor: remove AttachImage tui event (#3191)

TuiEvent is supposed to be purely events that come from the "driver",
i.e. events from the terminal. Everything app-specific should be an
AppEvent. In this case, it didn't need to be an event at all.

codex-rs/tui/src/app.rs

@@ -183,15 +183,6 @@ impl App {
                         },
                     )?;
                 }
-                TuiEvent::AttachImage {
-                    path,
-                    width,
-                    height,
-                    format_label,
-                } => {
-                    self.chat_widget
-                        .attach_image(path, width, height, format_label);
-                }
             }
         }
         Ok(true)