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/app-server-protocol/schema/json/v2/ThreadStartedNotification.json

@@ -14,6 +14,27 @@
     "AgentPath": {
       "type": "string"
     },
+    "AsyncUserInputQuestion": {
+      "additionalProperties": false,
+      "properties": {
+        "options": {
+          "items": {
+            "type": "string"
+          },
+          "type": [
+            "array",
+            "null"
+          ]
+        },
+        "title": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "title"
+      ],
+      "type": "object"
+    },
     "ByteRange": {
       "properties": {
         "end": {
@@ -1369,6 +1390,16 @@
               ],
               "default": null
             },
+            "questions": {
+              "default": null,
+              "items": {
+                "$ref": "#/definitions/AsyncUserInputQuestion"
+              },
+              "type": [
+                "array",
+                "null"
+              ]
+            },
             "text": {
               "type": "string"
             },