Handle undefined values before JSON serialization in code mode (#43873)

## Why

Passing `undefined` to a tool could fail JSON parsing because V8 stringifies it as the non-JSON text `undefined`.

## What changed

Return no JSON value for JavaScript `undefined`, so explicit `undefined` tool arguments behave like omitted arguments. Attempts to `store` an undefined value report the existing serializability error and preserve the previous stored value.

## Testing

Add regression coverage for storing `undefined` over an existing `null` value, and extend the current-time tool test to cover `{}`, omitted arguments, and explicit `undefined`.

GitOrigin-RevId: 071050ebb2ecc44c8f08453cf8523b4d21728c7e

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

@@ -297,6 +297,11 @@ pub(super) fn v8_value_to_json(
     scope: &mut v8::PinScope<'_, '_>,
     value: v8::Local<'_, v8::Value>,
 ) -> Result<Option<JsonValue>, String> {
+    // V8 stringifies undefined as the non-JSON text "undefined".
+    if value.is_undefined() {
+        return Ok(None);
+    }
+
     let tc = std::pin::pin!(v8::TryCatch::new(scope));
     let mut tc = tc.init();
     let Some(stringified) = v8::json::stringify(&tc, value) else {

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

@@ -474,6 +474,48 @@ async fn stored_values_are_shared_between_cells_but_not_sessions() {
     );
 }
 
+#[tokio::test]
+async fn storing_undefined_preserves_the_previous_value() {
+    let service = InProcessCodeModeSession::new();
+
+    let response = execute(
+        &service,
+        ExecuteRequest {
+            source: r#"
+store("key", null);
+try {
+  store("key", undefined);
+} catch (error) {
+  text(String(error));
+}
+text(load("key"));
+"#
+            .to_string(),
+            yield_time_ms: None,
+            ..execute_request("")
+        },
+    )
+    .await;
+
+    assert_eq!(
+        response,
+        RuntimeResponse::Result {
+            code_mode_host_duration: None,
+            cell_id: cell_id("1"),
+            content_items: vec![
+                FunctionCallOutputContentItem::InputText {
+                    text: "Unable to store \"key\". Only plain serializable objects can be stored."
+                        .to_string(),
+                },
+                FunctionCallOutputContentItem::InputText {
+                    text: "null".to_string(),
+                },
+            ],
+            error_text: None,
+        }
+    );
+}
+
 #[tokio::test]
 async fn shutdown_interrupts_cpu_bound_cells() {
     let service = InProcessCodeModeSession::new();