Double-check dangling pointers for getChildren() and getStyle()

[andreamancuso]

ChatGPT suggested the following:

const char* getChildren(int id) {
    Runner* pRunner = Runner::getInstance();
    static std::string result; // Static to avoid dangling pointers
    result = IntVectorToJson(pRunner->GetChildren(id)).dump();
    return result.c_str();
}

const char* getStyle() {
    Runner* pRunner = Runner::getInstance();
    static std::string result; // Static to avoid dangling pointers
    result = pRunner->GetStyle();
    return result.c_str();
}