Feat: support tree structured deep-research policy. (#12559)

### What problem does this PR solve?

#12558
### Type of change

- [x] New Feature (non-breaking change which adds functionality)

rag/nlp/search.py

@@ -382,6 +382,7 @@ class Dealer:
 
         # Ensure RERANK_LIMIT is multiple of page_size
         RERANK_LIMIT = math.ceil(64 / page_size) * page_size if page_size > 1 else 1
+        RERANK_LIMIT = max(30, RERANK_LIMIT)
         req = {
             "kb_ids": kb_ids,
             "doc_ids": doc_ids,