server : support preserving reasoning_content in assistant message (#18994)

* support reasoning_content input

* report template caps to webui

* add docs

* rm commented code

tools/server/server-context.h

@@ -22,6 +22,7 @@ struct server_context_meta {
 
     // chat params
     server_chat_params & chat_params;
+    std::map<std::string, bool> chat_template_caps;
 
     // tokens
     std::string bos_token_str;