Allow per-turn overrides for direct-only tool namespaces (#59481)

## What changed

- Add `direct_only_tool_namespaces` to `TurnStartOptions`. Omission inherits the thread configuration; an empty list clears direct-only routing for that turn. Steering ignores the option.
- Persist explicit overrides in turn context and restore them when recovering an interrupted turn, including turns started by standalone tool output.
- Keep subsequent turns and spawned or resumed child agents on the thread's routing defaults.

## Testing

Add coverage for routing changes across turns in Responses and Responses Lite, recovery without carrying overrides into subsequent turns, and child-agent inheritance.

GitOrigin-RevId: cea1a997424572d4cd372cd15b098626904c6796

codex-rs/core/src/tools/handlers/multi_agents_tests.rs

@@ -4689,6 +4689,41 @@ async fn build_agent_spawn_config_uses_captured_step_settings_and_turn_context_v
     assert_eq!(config, expected);
 }
 
+#[tokio::test]
+async fn child_agents_inherit_thread_routing_when_parent_overrides_it_for_one_turn() {
+    let home = tempfile::tempdir().expect("create test home");
+    let (session, _, _events) = make_session_and_context_with_auth_config_home_and_rx(
+        CodexAuth::from_api_key("test"),
+        Vec::new(),
+        home.path(),
+        |config| config.code_mode.direct_only_tool_namespaces = vec!["thread_tools".into()],
+    )
+    .await;
+    let turn = session
+        .new_turn_with_default_settings(
+            "parent-turn".into(),
+            crate::session::turn_context::NewTurnContextOptions {
+                direct_only_tool_namespaces: Some(vec!["user_message".into()]),
+                ..Default::default()
+            },
+        )
+        .await;
+    assert_eq!(
+        turn.config.code_mode.direct_only_tool_namespaces,
+        ["user_message"]
+    );
+    let resumed = build_agent_resume_config(&turn).expect("resume config");
+    let step = StepContext::for_test(turn);
+    let spawned =
+        build_agent_spawn_config(&BaseInstructions::default(), &step).expect("spawn config");
+    for config in [spawned, resumed] {
+        assert_eq!(
+            config.code_mode.direct_only_tool_namespaces,
+            ["thread_tools"]
+        );
+    }
+}
+
 #[tokio::test]
 async fn build_agent_resume_config_clears_base_instructions() {
     let (_session, mut turn) = make_session_and_context().await;