chore: handle all cases for EventMsg (#936)

For now, this removes the `#[non_exhaustive]` directive on `EventMsg` so
that we are forced to handle all `EventMsg` by default. (We may revisit
this if/when we publish `core/` as a `lib` crate.) For now, it is
helpful to have this as a forcing function because we have effectively
two UIs (`tui` and `exec`) and usually when we add a new variant to
`EventMsg`, we want to be sure that we update both.

codex-rs/core/tests/previous_response_id.rs

@@ -155,7 +155,9 @@ async fn keeps_previous_response_id_between_tasks() {
             EventMsg::Error(ErrorEvent { message }) => {
                 panic!("unexpected error: {message}")
             }
-            _ => (),
+            _ => {
+                // Ignore other events.
+            }
         }
     }
 }