Release db.session connection before workflow new thread long time operation (#21726)

Co-authored-by: 李强04 <liqiang04@gaotu.cn>

api/core/app/apps/workflow/app_generator.py

@@ -219,6 +219,9 @@ class WorkflowAppGenerator(BaseAppGenerator):
         # new thread with request context and contextvars
         context = contextvars.copy_context()
 
+        # release database connection, because the following new thread operations may take a long time
+        db.session.close()
+
         worker_thread = threading.Thread(
             target=self._generate_worker,
             kwargs={