get_reranker() acquires the module-level _lock and then, on first load,
calls unload() to clear any stale state before _load() instantiates the
CrossEncoder. unload() acquires the same _lock — but threading.Lock is
non-reentrant, so the second acquisition by the holding thread blocked
forever. Symptom: rerank=True hung the search_knowledge_base tool with
no further log output past 'rerank entered'.
Switch to threading.RLock so the same thread can re-enter without
blocking. unload()'s independent callers still work the same way; the
only behaviour change is that re-entrant acquisition from one thread
now succeeds.
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