server : fix validate_model_chat_template

Author: jhen0409

examples/server/server.cpp

@@ -657,9 +657,11 @@ struct server_context {
     bool validate_model_chat_template() const {
         llama_chat_message chat[] = {{"user", "test"}};
 
-        const int res = llama_chat_apply_template(model, nullptr, chat, 1, true, nullptr, 0);
+        std::vector<char> model_template(2048, 0); // longest known template is about 1200 bytes
+        std::string template_key = "tokenizer.chat_template";
+        int32_t res = llama_model_meta_val_str(model, template_key.c_str(), model_template.data(), model_template.size());
 
-        return res > 0;
+        return res >= 0;
     }
 
     void init() {