Use Option symbol for mac key hints (#5582)

## Summary
- show the Option (⌥) symbol in key hints when the TUI is built for
macOS so the shortcut text matches the platform terminology

## Testing
- cargo test -p codex-tui

------
https://chatgpt.com/codex/tasks/task_i_68fab7505530832992780a9e13fb707b

codex-rs/tui/src/key_hint.rs

@@ -6,6 +6,9 @@ use ratatui::style::Style;
 use ratatui::style::Stylize;
 use ratatui::text::Span;
 
+#[cfg(target_os = "macos")]
+const ALT_PREFIX: &str = "⌥ + ";
+#[cfg(not(target_os = "macos"))]
 const ALT_PREFIX: &str = "alt + ";
 const CTRL_PREFIX: &str = "ctrl + ";
 const SHIFT_PREFIX: &str = "shift + ";

codex-rs/tui/src/snapshots/codex_tui__status_indicator_widget__tests__renders_with_queued_messages@macos.snap

@@ -0,0 +1,13 @@
+--- 
+source: tui/src/status_indicator_widget.rs
+assertion_line: 289
+expression: terminal.backend()
+---
+"• Working (0s • esc to interrupt)                                               "
+"                                                                                "
+" ↳ first                                                                        "
+" ↳ second                                                                       "
+"   ⌥ + ↑ edit                                                                   "
+"                                                                                "
+"                                                                                "
+"                                                                                "

codex-rs/tui/src/status_indicator_widget.rs

@@ -284,6 +284,11 @@ mod tests {
         terminal
             .draw(|f| w.render_ref(f.area(), f.buffer_mut()))
             .expect("draw");
+        #[cfg(target_os = "macos")]
+        insta::with_settings!({ snapshot_suffix => "macos" }, {
+            insta::assert_snapshot!(terminal.backend());
+        });
+        #[cfg(not(target_os = "macos"))]
         insta::assert_snapshot!(terminal.backend());
     }