feat(cache): 增加 LLM 相似性检查功能并优化缓存机制

- 在 embedding 缓存配置中添加 use_llm_check 参数
- 实现 LLM 相似性检查逻辑，作为缓存命中的二次验证- 优化 naive 模式的缓存处理流程
- 调整缓存数据结构，移除不必要的 model 字段

lightrag/prompt.py

@@ -261,3 +261,22 @@ Do not include information where the supporting evidence for it is not provided.
 
 Add sections and commentary to the response as appropriate for the length and format. Style the response in markdown.
 """
+
+PROMPTS[
+    "similarity_check"
+] = """Please analyze the similarity between these two questions:
+
+Question 1: {original_prompt}
+Question 2: {cached_prompt}
+
+Please evaluate:
+1. Whether these two questions are semantically similar
+2. Whether the answer to Question 2 can be used to answer Question 1
+
+Please provide a similarity score between 0 and 1, where:
+0: Completely unrelated or answer cannot be reused
+1: Identical and answer can be directly reused
+0.5: Partially related and answer needs modification to be used
+
+Return only a number between 0-1, without any additional content.
+"""