Make random_string() thread-safe (#2110)

By making the random engine thread_local, each thread now has its own
independent random sequence, ensuring safe concurrent access.
Additionally, using an immediately invoked lambda expression to
initialize the engine eliminates the need for separate static seed
variables.

httplib.h

@@ -5100,16 +5100,15 @@ inline std::string random_string(size_t length) {
   constexpr const char data[] =
       "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
 
-  // std::random_device might actually be deterministic on some
-  // platforms, but due to lack of support in the c++ standard library,
-  // doing better requires either some ugly hacks or breaking portability.
-  static std::random_device seed_gen;
-
-  // Request 128 bits of entropy for initialization
-  static std::seed_seq seed_sequence{seed_gen(), seed_gen(), seed_gen(),
-                                     seed_gen()};
-
-  static std::mt19937 engine(seed_sequence);
+  static thread_local std::mt19937 engine([]() {
+    // std::random_device might actually be deterministic on some
+    // platforms, but due to lack of support in the c++ standard library,
+    // doing better requires either some ugly hacks or breaking portability.
+    std::random_device seed_gen;
+    // Request 128 bits of entropy for initialization
+    std::seed_seq seed_sequence{seed_gen(), seed_gen(), seed_gen(), seed_gen()};
+    return std::mt19937(seed_sequence);
+  }());
 
   std::string result;
   for (size_t i = 0; i < length; i++) {