[9.1] Fix deadlock in ThreadPoolMergeScheduler when a failing merge closes the IndexWriter (#134656)#135173

tlrx merged 1 commit intoelastic:9.1elastic/elasticsearch:9.1from

tlrx:2025/09/09/ES-12664-9.1tlrx/elasticsearch:2025/09/09/ES-12664-9.1Copy head branch name to clipboard