Fix tokenizer in cpp (#13735)

### What problem does this PR solve?

Tokenzier in Infinity is modified in
https://github.com/infiniflow/infinity/pull/3330, sync the code change
to cpp files in ragflow

### Type of change

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

internal/cpp/rag_analyzer.h

@@ -136,7 +136,7 @@ public:
     [[nodiscard]] std::vector<std::pair<std::vector<std::string_view>, double>> GetBestTokensTopN(
         std::string_view chars, uint32_t n) const;
 
-    static const size_t term_string_buffer_limit_ = 4096 * 3;
+    static constexpr size_t term_string_buffer_limit_ = 4096 * 3;
 
     std::string dict_path_;
 
@@ -152,6 +152,8 @@ public:
 
     OpenCC* opencc_{nullptr};
 
+    mutable std::vector<char> lowercase_string_buffer_;
+
     bool fine_grained_{false};
 
     bool enable_position_{false};