Fix:Improve float operation when rerank (#8963)

### What problem does this PR solve?

https://github.com/infiniflow/ragflow/issues/8915

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/llm/rerank_model.py

@@ -358,7 +358,7 @@ class OpenAI_APIRerank(Base):
         max_rank = np.max(rank)
 
         # Avoid division by zero if all ranks are identical
-        if max_rank - min_rank != 0:
+        if np.isclose(min_rank, max_rank, atol=1e-3):
             rank = (rank - min_rank) / (max_rank - min_rank)
         else:
             rank = np.zeros_like(rank)