isJsonObject function
Narrows value to a plain JSON object: a Map keyed by String that is
not a list.
This is the Dart twin of the sibling isRecord guard. Dart's type system
already separates maps from lists, so the check is a single is test rather
than the runtime null/array probing the TypeScript sibling needs.
Implementation
bool isJsonObject(Object? value) => value is Map<String, Object?>;