Register the async message feature flag (#39288)

## What changed

- Add the under-development `send_async_message` feature, disabled by default.
- Expose the feature key in the generated configuration schema.

GitOrigin-RevId: dd27483f2ef802dba49e47f3ce03146ee6a58f1e

codex-rs/features/src/lib.rs

@@ -262,6 +262,8 @@ pub enum Feature {
     MentionsV2,
     /// Allow request_user_input in Default collaboration mode.
     DefaultModeRequestUserInput,
+    /// Allow sending a user-visible message without ending the current turn.
+    SendAsyncMessage,
     /// Enable automatic review for approval prompts.
     GuardianApproval,
     /// Reuse encrypted parent compaction when restarting Guardian review sessions.
@@ -1350,6 +1352,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::SendAsyncMessage,
+        key: "send_async_message",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::TerminalVisualizationInstructions,
         key: "terminal_visualization_instructions",