Remove the feature gate for async user messages (#39452)

## What changed

- Expose `send_user_message_async` to root agents whenever the selected model advertises support for it.
- Retain `send_async_message` as a removed compatibility flag so existing configuration is accepted without controlling tool availability.
- Update the async message test to cover model-driven tool exposure without enabling the legacy flag.

GitOrigin-RevId: 558b427c2b203a057dd7c836e0ec702ae6134346

codex-rs/core/src/tools/spec_plan.rs

@@ -1063,7 +1063,6 @@ fn add_core_utility_tools(context: &CoreToolPlanContext<'_>, registry: &mut Tool
     }
 
     if !turn_context.session_source.is_non_root_agent()
-        && features.enabled(Feature::SendAsyncMessage)
         && turn_context
             .model_info
             .experimental_supported_tools

codex-rs/core/tests/suite/send_user_message_async.rs

@@ -1,6 +1,5 @@
 use anyhow::Result;
 use codex_core::TurnInputRequest;
-use codex_features::Feature;
 use codex_protocol::items::AgentMessageContent;
 use codex_protocol::items::AgentMessageDelivery;
 use codex_protocol::items::AgentMessageItem;
@@ -57,12 +56,6 @@ async fn send_user_message_async_emits_item_and_does_not_end_the_turn() -> Resul
                 .experimental_supported_tools
                 .push("send_user_message_async".to_string());
         })
-        .with_config(|config| {
-            config
-                .features
-                .enable(Feature::SendAsyncMessage)
-                .expect("test config should allow enabling async messages");
-        })
         .build_with_auto_env(&server)
         .await?;