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

tests/peg-parser/tests.h

@@ -1,7 +1,7 @@
 #pragma once
 
 // Common includes for all test files
-#include <nlohmann/json.hpp>
+#include "json.h"
 #include <string>
 #include <vector>
 
@@ -11,9 +11,9 @@
 #include "simple-tokenize.h"
 
 struct bench_tool_call {
-    std::string            id;
-    std::string            name;
-    nlohmann::ordered_json args;
+    std::string id;
+    std::string name;
+    common_json args;
 };
 
 // Test function declarations