Add structured asynchronous user input requests (#42178)

## What changed

- Replace `send_user_message_async` with `request_user_input_async`, accepting one or more questions with optional suggested answers while allowing the turn to continue.
- Attach structured question metadata to asynchronous agent messages while retaining readable fallback text, and preserve it through app-server events, thread history, and generated schemas.
- Continue enabling the new tool for model catalogs that advertise either the old or new tool name.

## Testing

- Cover tool registration, question validation, emitted items, continued turn execution, history preservation, and backwards-compatible deserialization.

GitOrigin-RevId: ffc49aebde2e854c3c50a122aa52805fec3fc6b3

codex-rs/core/src/session/turn.rs

@@ -2088,6 +2088,7 @@ async fn emit_agent_message_in_plan_mode(
                     phase: None,
                     memory_citation: None,
                     delivery: None,
+                    questions: None,
                 })
             });
         sess.emit_turn_item_started(turn_context, &start_item).await;