style: apply concat() suggestion and rerun make amalgamate

- Replaced std::string concatenation with concat() per maintainer feedback
- Added missing newline in unit-serialize_enum_strict.cpp
- Regenerated single-header files using make amalgamate for proper indentation

Signed-off-by: Ash-Jose <ashlinjose986@outlook.com>

Author: Ash-Jose

include/nlohmann/detail/macro_scope.hpp

@@ -289,8 +289,8 @@
         {                                                                                       \
             throw ::nlohmann::detail::type_error::create(                                       \
                 302,                                                                            \
-                std::string("invalid value for ") + #ENUM_TYPE + ": " + j.dump(),               \
-                nullptr);                                                                       \
+                concat("invalid value for ", #ENUM_TYPE, ": ", j.dump()),                       \
+                j);                                                                             \
         }                                                                                       \
         e = it->first;                                                                          \
     }

single_include/nlohmann/json.hpp

@@ -2653,8 +2653,8 @@ JSON_HEDLEY_DIAGNOSTIC_POP
         {                                                                                       \
             throw ::nlohmann::detail::type_error::create(                                       \
                 302,                                                                            \
-                std::string("invalid value for ") + #ENUM_TYPE + ": " + j.dump(),               \
-                nullptr);                                                                       \
+                concat("invalid value for ", #ENUM_TYPE, ": ", j.dump()),                       \
+                j);                                                                             \
         }                                                                                       \
         e = it->first;                                                                          \
     }

tests/src/unit-serialize_enum_strict.cpp

@@ -26,4 +26,4 @@ TEST_CASE("NLOHMANN_JSON_SERIALIZE_ENUM_STRICT still deserializes valid values")
     json j = "green";
     auto c = j.get<ns::Color>();
     CHECK(c == ns::Color::green);
-}
+}