fix: optimization: PVS-Studio warning V831

Replaced 'at()' method with 'operator[]' to improve performance.

The 'at()' method performs bounds checking, which can introduce overhead. Using 'operator[]' bypasses this check and can improve performance when you are certain that the index is within bounds.

Reported by: PVS-Studio
Signed-off-by: Vyacheslav Ivanov <islavaivanov76@gmail.com>

src/util/listutil.cpp

@@ -7,7 +7,7 @@ std::string util::to_string(const std::vector<std::string>& vec) {
     std::stringstream ss;
     ss << "[";
     for (size_t i = 0; i < vec.size(); i++) {
-        ss << util::quote(vec.at(i));
+        ss << util::quote(vec[1]);
         if (i < vec.size()-1) {
             ss << ", ";
         }