Clarify external editor env var message (#10030)

### Motivation
- Improve UX by making it explicit that `VISUAL`/`EDITOR` must be set
before launching Codex, not during a running session.

### Description
- Update the external editor error text in `codex-rs/tui/src/app.rs` to:
`"Cannot open external editor: set $VISUAL or $EDITOR before starting
Codex."` and run `just fmt` to apply formatting.

### Testing
- Ran `just fmt` successfully; attempted `cargo test -p codex-tui` but
it failed due to network errors when fetching git dependencies (tests
did not complete).

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_6972c2c984948329b1a37d5c5839aff3)

codex-rs/tui/src/app.rs

@@ -2268,8 +2268,9 @@ impl App {
             Err(external_editor::EditorError::MissingEditor) => {
                 self.chat_widget
                     .add_to_history(history_cell::new_error_event(
-                        "Cannot open external editor: set $VISUAL or $EDITOR".to_string(),
-                    ));
+                    "Cannot open external editor: set $VISUAL or $EDITOR before starting Codex."
+                        .to_string(),
+                ));
                 self.reset_external_editor_state(tui);
                 return;
             }