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/external-agent-migration/src/sessions/export.rs

@@ -136,6 +136,7 @@ pub(super) fn rollout_items_from_messages(messages: Vec<ConversationMessage>) ->
                         phase: None,
                         memory_citation: None,
                         delivery: None,
+                        questions: None,
                     },
                 )));
                 items.push(RolloutItem::ResponseItem(response_item(message).into()));
@@ -158,6 +159,7 @@ fn external_session_imported_marker_item() -> RolloutItem {
         phase: None,
         memory_citation: None,
         delivery: None,
+        questions: None,
     }))
 }
 
@@ -254,6 +256,7 @@ mod tests {
                 phase: None,
                 memory_citation: None,
                 delivery: None,
+                questions: None,
             }
         );
     }
@@ -287,6 +290,7 @@ mod tests {
                 phase: None,
                 memory_citation: None,
                 delivery: None,
+                questions: None,
             })
         );
         let last_turn_complete = imported