Test text stringify errors in the code mode runtime (#39505)

## What changed

Move circular-value coverage for the `text()` helper from the core integration
suite to the in-process code mode runtime tests. Verify that stringification
returns no content and surfaces the V8 circular-structure error without needing
a mock server or network access.

GitOrigin-RevId: d506591af5ae8ce0a8c7305d633a7b0f2e458335

codex-rs/code-mode-runtime/src/service_tests.rs

@@ -937,6 +937,47 @@ text(JSON.stringify(returnsUndefined));
     );
 }
 
+#[tokio::test]
+async fn text_helper_surfaces_stringify_errors() {
+    let service = InProcessCodeModeSession::new();
+
+    let response = execute(
+        &service,
+        ExecuteRequest {
+            source: r#"
+const circular = {};
+circular.self = circular;
+text(circular);
+"#
+            .to_string(),
+            yield_time_ms: None,
+            ..execute_request("")
+        },
+    )
+    .await;
+
+    let RuntimeResponse::Result {
+        error_text: Some(error_text),
+        ..
+    } = &response
+    else {
+        panic!("circular stringify unexpectedly succeeded: {response:?}");
+    };
+    assert!(
+        error_text.contains("Converting circular structure to JSON"),
+        "unexpected circular stringify error: {error_text}"
+    );
+    let error_text = error_text.clone();
+    assert_eq!(
+        response,
+        RuntimeResponse::Result {
+            cell_id: cell_id("1"),
+            content_items: Vec::new(),
+            error_text: Some(error_text),
+        }
+    );
+}
+
 #[tokio::test]
 async fn audio_helper_accepts_audio_url_object_and_raw_mcp_audio_block() {
     let service = InProcessCodeModeSession::new();