fix: int128 ostream helper

utils/int128.h

@@ -2,12 +2,33 @@
 #include <ostream>
 #include <string>
 
+/**
+ * Prints an 128-bit integer to an output stream.
+ */
 std::ostream& operator<<(std::ostream& __ostream, __int128 __n) {
     std::string __o;
+
+    if (__n == 0) {
+        return __ostream << 0;
+    }
+
+    bool is_negative = false;
+
+    if (__n < 0) {
+        is_negative = true;
+        __n = -__n;
+    }
+
     while (__n) {
         __o.push_back(__n % 10 + '0');
         __n /= 10;
     }
+
     std::reverse(__o.begin(), __o.end());
+
+    if (is_negative) {
+        __ostream << '-';
+    }
+
     return __ostream << __o;
 }