common: add json.h abstraction (#27511)

* add common/json

* migrate common

* adapt jinja

* migrate server

* big wip

* migrate tests

* wip

* revert some excessive changes

* wip

* wip 2

* revert redundant changes

* fix server crash

* various fixes

* fix ci

* harden a bit

* clean up

* rm json-shim

* add some comments

* rm redundant decl

tools/server/server-chat.h

@@ -6,9 +6,7 @@
 #include "server-common.h"
 #include "server-http.h"
 
-#include <nlohmann/json_fwd.hpp>
-
-using json = nlohmann::ordered_json;
+#include "json.h"
 
 // Convert OpenAI Responses API format to OpenAI Chat Completions API format
 json server_chat_convert_responses_to_chatcmpl(const json & body);