perf: O(1) end-tag detection via tail slice

Author: Algorithm5838

backend/open_webui/utils/middleware.py

@@ -3416,7 +3416,7 @@ def set_last_text(out, text):
                     else:
                         block_content = get_last_text(output)
 
-                    if re.search(end_tag_pattern, block_content):
+                    if end_tag in block_content[-512:]:
                         end_flag = True
 
                         # Strip start and end tags from content