feat: ctrl-d only exits when there is no user input (#1589)

While this does make it so that `ctrl-d` will not exit Codex when the
composer is not empty, `ctrl-d` will still exit Codex if it is in the
"working" state.

Fixes https://github.com/openai/codex/issues/1443.

codex-rs/tui/src/bottom_pane/chat_composer_history.rs

@@ -72,8 +72,7 @@ impl ChatComposerHistory {
             return false;
         }
 
-        let lines = textarea.lines();
-        if lines.len() == 1 && lines[0].is_empty() {
+        if textarea.is_empty() {
             return true;
         }
 
@@ -85,6 +84,7 @@ impl ChatComposerHistory {
             return false;
         }
 
+        let lines = textarea.lines();
         matches!(&self.last_history_text, Some(prev) if prev == &lines.join("\n"))
     }