Let dynamic threads wait for work instead of exiting immediately

Previously, dynamic threads exited as soon as their current task
completed and the queue was empty. This caused excessive thread
creation/destruction under bursty or long-lived workloads (e.g., SSE
streaming), degrading tail latency. Now dynamic threads loop back and
wait for CPPHTTPLIB_THREAD_POOL_IDLE_TIMEOUT (3s) before exiting,
allowing them to be reused for subsequent tasks.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

httplib.h

@@ -10136,15 +10136,6 @@ inline void ThreadPool::worker(bool is_dynamic) {
 
     assert(true == static_cast<bool>(fn));
     fn();
-
-    // Dynamic thread: exit if queue is empty after task completion
-    if (is_dynamic) {
-      std::unique_lock<std::mutex> lock(mutex_);
-      if (jobs_.empty()) {
-        move_to_finished(std::this_thread::get_id());
-        break;
-      }
-    }
   }
 
 #if defined(CPPHTTPLIB_OPENSSL_SUPPORT) && !defined(OPENSSL_IS_BORINGSSL) &&   \