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/thread-store/src/local/mod.rs

@@ -1043,6 +1043,7 @@ mod tests {
                     phase: Some(MessagePhase::Commentary),
                     memory_citation: None,
                     delivery: None,
+                    questions: None,
                 })),
                 RolloutItem::ResponseItem(
                     ResponseItem::FunctionCallOutput {

codex-rs/thread-store/src/local/rollout_migration/legacy_event.rs

@@ -95,6 +95,7 @@ pub(super) fn completed_item(
                 phase: event.phase.clone(),
                 memory_citation: event.memory_citation.clone(),
                 delivery: event.delivery,
+                questions: event.questions.clone(),
             }),
             None,
         )),

codex-rs/thread-store/src/local/rollout_migration_tests.rs

@@ -160,6 +160,7 @@ fn agent_message(text: &str) -> RolloutItem {
         phase: None,
         memory_citation: None,
         delivery: None,
+        questions: None,
     }))
 }
 

codex-rs/thread-store/src/local/thread_history_materialization_tests.rs

@@ -232,6 +232,7 @@ async fn split_homes_support_backfill_listing_and_paginated_history() {
                         phase: None,
                         memory_citation: None,
                         delivery: None,
+                        questions: None,
                     }),
                 ),
                 turn_completed("turn-1"),
@@ -325,6 +326,7 @@ async fn paginated_live_append_materializes_turn_items_and_state() {
                         phase: None,
                         memory_citation: None,
                         delivery: None,
+                        questions: None,
                     }),
                 ),
                 turn_completed("turn-1"),
@@ -480,6 +482,7 @@ async fn paginated_realtime_items_materialize_separately_in_rollout_order() {
                         phase: None,
                         memory_citation: None,
                         delivery: None,
+                        questions: None,
                     }),
                 ),
                 RolloutItem::RealtimeItem(RealtimeItem {
@@ -2669,6 +2672,7 @@ fn agent_message(id: &str, phase: MessagePhase) -> TurnItem {
         phase: Some(phase),
         memory_citation: None,
         delivery: None,
+        questions: None,
     })
 }