Add a feature flag for asynchronous user messages (#45124)

## What changed

Add the disabled-by-default `send_message_to_user_async` feature flag so root agents can use the tool without model catalog support. Preserve catalog-based opt-in and keep the tool unavailable to subagents. Register the flag as under development and expose it in the configuration schema.

## Testing

Extend integration tests to cover feature and catalog opt-ins, deduplication when both are enabled, subagent exclusion, and the retired `send_async_message` flag. Check that either opt-in allows messages to be emitted without ending the turn.

GitOrigin-RevId: 3f53181ab074b0432c795550de73a9b1da7caf15

codex-rs/core/config.schema.json

@@ -971,6 +971,9 @@
             "send_async_message": {
               "type": "boolean"
             },
+            "send_message_to_user_async": {
+              "type": "boolean"
+            },
             "shell_snapshot": {
               "type": "boolean"
             },
@@ -6586,6 +6589,9 @@
         "send_async_message": {
           "type": "boolean"
         },
+        "send_message_to_user_async": {
+          "type": "boolean"
+        },
         "shell_snapshot": {
           "type": "boolean"
         },