fix: avoid copy-assign on const-key pairs in from_json for map types

std::transform + std::inserter on vector-backed ordered_map triggers element shifting code in GCC's <algorithm>, which requires pair::operator=.
That is deleted when the key is const (e.g. ordered_map<string, string>).

Replace with a range-for + emplace so no assignment is ever needed. Fix for the bug #5122

Author: ssam18

include/nlohmann/detail/conversions/from_json.hpp

@@ -386,14 +386,10 @@ inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
 
     ConstructibleObjectType ret;
     const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    using value_type = typename ConstructibleObjectType::value_type;
-    std::transform(
-        inner_object->begin(), inner_object->end(),
-        std::inserter(ret, ret.begin()),
-        [](typename BasicJsonType::object_t::value_type const & p)
+    for (const auto& p : *inner_object)
     {
-        return value_type(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    });
+        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
+    }
     obj = std::move(ret);
 }