Keep backtrack Esc hint gated on empty composer (#5076)

## Summary
- only prime backtrack and show the ESC hint when the composer is empty
- keep the composer-side ESC hint unchanged when drafts or attachments
exist and cover it with a regression test

Fixes #5030

------
https://chatgpt.com/codex/tasks/task_i_68e95ba59cd8832caec8e72ae2efeb55

codex-rs/tui/src/app.rs

@@ -427,8 +427,9 @@ impl App {
                 tui.frame_requester().schedule_frame();
             }
             // Esc primes/advances backtracking only in normal (not working) mode
-            // with an empty composer. In any other state, forward Esc so the
-            // active UI (e.g. status indicator, modals, popups) handles it.
+            // with the composer focused and empty. In any other state, forward
+            // Esc so the active UI (e.g. status indicator, modals, popups)
+            // handles it.
             KeyEvent {
                 code: KeyCode::Esc,
                 kind: KeyEventKind::Press | KeyEventKind::Repeat,