Expose the model selected for a running turn's next step (#50128)

## What changed

Add `CodexThread::current_turn_model` to return the model slug selected for the named running turn's next step, independently of settings for future turns. Return `None` when the turn ID does not match, the task is cancelled, or the turn is no longer running.

## Testing

Extend the step-settings test to verify that the accessor ignores model changes for future turns, reflects active-turn model updates, and returns `None` for a mismatched ID or a completed turn.

GitOrigin-RevId: 2cae7c07fa761ed4d64befaa3419a2f9bce4b161

codex-rs/core/src/codex_thread.rs

@@ -304,6 +304,17 @@ impl CodexThread {
         task.turn_context.extension_data.get::<T>()
     }
 
+    /// Returns the model selected for the named running turn's next step, not future turns.
+    /// Returns `None` if that turn is no longer running.
+    pub async fn current_turn_model(&self, expected_turn_id: &str) -> Option<String> {
+        let active = self.session.active_turn.lock().await;
+        let task = active.as_ref()?.task.as_ref()?;
+        if task.turn_context.sub_id != expected_turn_id || task.cancellation_token.is_cancelled() {
+            return None;
+        }
+        Some(task.turn_context.capture_current_model_info().slug.clone())
+    }
+
     pub async fn shutdown_and_wait(&self) -> CodexResult<()> {
         self.io.shutdown_and_wait().await
     }

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

@@ -1948,6 +1948,7 @@ async fn sparse_updates_preserve_divergent_active_and_future_models() -> Result<
     .await;
     let test = step_settings_test().build_with_auto_env(&server).await?;
     let request = start_paused_turn(&test.codex).await?;
+    assert_eq!(test.codex.current_turn_model("other-turn").await, None);
 
     core_test_support::submit_thread_settings(
         &test.codex,
@@ -1957,6 +1958,10 @@ async fn sparse_updates_preserve_divergent_active_and_future_models() -> Result<
         },
     )
     .await?;
+    assert_eq!(
+        test.codex.current_turn_model(&request.turn_id).await,
+        Some(MODEL_A.to_string())
+    );
     apply_turn_settings(
         &test.codex,
         &request.turn_id,
@@ -1967,6 +1972,10 @@ async fn sparse_updates_preserve_divergent_active_and_future_models() -> Result<
         },
     )
     .await?;
+    assert_eq!(
+        test.codex.current_turn_model(&request.turn_id).await,
+        Some(MODEL_C.to_string())
+    );
     answer_paused_turn(&test.codex, &request.turn_id).await?;
     let second_request = wait_for_event_match(&test.codex, |event| match event {
         EventMsg::RequestUserInput(request) => Some(request.clone()),
@@ -2013,6 +2022,7 @@ async fn sparse_updates_preserve_divergent_active_and_future_models() -> Result<
         matches!(event, EventMsg::TurnComplete(_))
     })
     .await;
+    assert_eq!(test.codex.current_turn_model(&request.turn_id).await, None);
     test.submit_text_turn("start the next turn").await?;
 
     let requests = response_mock.requests();