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/agent/child_config.rs

@@ -143,6 +143,12 @@ fn build_agent_shared_config(turn: &TurnContext) -> Result<Config, String> {
     // Preserve activation for history forks without freezing the parent's model-owned prompts.
     // Fresh child startup restores configured preferences from the retained snapshot.
     config.token_budget = turn.configured_token_budget.clone();
+    if let Some(thread_namespaces) = &turn.overridden_direct_only_tool_namespaces {
+        config
+            .code_mode
+            .direct_only_tool_namespaces
+            .clone_from(thread_namespaces);
+    }
     config.model = Some(turn.model_info().slug.clone());
     config.model_provider = turn.provider.info().clone();
     config.model_reasoning_effort = turn