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/tui/src/resume_picker_transcript_preview_tests.rs

@@ -42,6 +42,7 @@ fn rollout_agent_message(text: &str) -> RolloutItem {
         phase: None,
         memory_citation: None,
         delivery: None,
+        questions: None,
     }))
 }
 
@@ -269,6 +270,7 @@ fn transcript_preview_reverse_scan_stops_before_older_items() {
         phase: None,
         memory_citation: None,
         delivery: None,
+        questions: None,
     };
     let mut lines = Vec::new();