feat(tui): support Alt-d delete-forward-word (#12455)

Alt-d should delete the next word. It didn’t. Now it does. Added a small
test so it stays that way.

Details:
File updated:
[codex-rs/tui/src/bottom_pane/textarea.rs](./codex-rs/tui/src/bottom_pane/textarea.rs)
Test added: delete_forward_word_alt_d — verifies Alt-d deletes the next
word and keeps the cursor position correct.

Solves  Issue #12453

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

@@ -322,7 +322,12 @@ impl TextArea {
                 code: KeyCode::Delete,
                 modifiers: KeyModifiers::ALT,
                 ..
-            }  => self.delete_forward_word(),
+            }
+            | KeyEvent {
+                code: KeyCode::Char('d'),
+                modifiers: KeyModifiers::ALT,
+                ..
+            } => self.delete_forward_word(),
             KeyEvent {
                 code: KeyCode::Delete,
                 ..
@@ -1763,6 +1768,15 @@ mod tests {
         assert_eq!(t.cursor(), 0);
     }
 
+    #[test]
+    fn delete_forward_word_alt_d() {
+        let mut t = ta_with("hello world");
+        t.set_cursor(6);
+        t.input(KeyEvent::new(KeyCode::Char('d'), KeyModifiers::ALT));
+        pretty_assertions::assert_eq!(t.text(), "hello ");
+        pretty_assertions::assert_eq!(t.cursor(), 6);
+    }
+
     #[test]
     fn control_h_backspace() {
         // Test Ctrl+H as backspace