Add turn-aware response item injection (#38047)

## What changed

- Add `inject_response_items_for_turn` for callers that inject Responses API
  items immediately before admitting user input. It defers the rollout flush so
  the injected items and user input are persisted together by turn admission.
- Keep `inject_response_items` behavior unchanged by flushing after it delegates
  to the new helper.
- Update the turn-start persistence test to exercise the turn-aware path.

GitOrigin-RevId: 8349b65f937af2b2a95d36b3fd73d862d38e5fd9

codex-rs/core/src/codex_thread.rs

@@ -593,6 +593,20 @@ impl CodexThread {
 
     /// Record raw Responses API items without starting a new turn.
     pub async fn inject_response_items(&self, items: Vec<ResponseItem>) -> CodexResult<()> {
+        self.inject_response_items_for_turn(items).await?;
+        self.session.flush_rollout().await?;
+        Ok(())
+    }
+
+    /// Record raw Responses API items immediately before admitting a user turn.
+    ///
+    /// The caller must submit the associated user input while retaining its
+    /// thread-operation lock. The subsequent turn persistence includes both
+    /// these items and the user input, without an independent rollout flush.
+    pub async fn inject_response_items_for_turn(
+        &self,
+        items: Vec<ResponseItem>,
+    ) -> CodexResult<()> {
         if items.is_empty() {
             return Err(CodexErr::InvalidRequest(
                 "items must not be empty".to_string(),
@@ -613,7 +627,6 @@ impl CodexThread {
         self.session
             .inject_no_new_turn(items, Some(turn_context.as_ref()))
             .await;
-        self.session.flush_rollout().await?;
         Ok(())
     }
 

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

@@ -54,7 +54,7 @@ async fn default_turn_start_persists_developer_and_user_input_before_model_reque
         .await
         .expect("build default thread-store session");
     test.codex
-        .inject_response_items(vec![ResponseItem::Message {
+        .inject_response_items_for_turn(vec![ResponseItem::Message {
             id: None,
             role: "developer".to_string(),
             content: vec![ContentItem::InputText {