server: fix duplicate HTTP headers in multiple models mode (#17698)

* llama-server: fix duplicate HTTP headers in multiple models mode (#17693)

* llama-server: address review feedback from ngxson

- restrict scope of header after std::move
- simplify header check (remove unordered_set)

tools/server/server-models.h

@@ -170,5 +170,6 @@ private:
         std::map<std::string, std::string> headers;
         int status = 0;
         std::string data;
+        std::string content_type;
     };
 };