Fix format_as wrapping strings in double quotes + add unit tests

[Copilot]

format_as was using j.dump() unconditionally, which wraps JSON string values in double quotes. This caused include path lookups to fail (e.g. "msg"/UUID.json instead of msg/UUID.json).

Changes

• Fix format_as(const json&): use j.get<std::string>() for string values, fall back to j.dump() for all other types
• Fix format_as(const detail::iter_impl<const json>&): delegate to the value overload via format_as(*j) for consistency
• Add unit tests in rosidlcpp_generator_core/test/unit/ covering both overloads: string values are returned unquoted, non-string types (int, bool, null, array, object) continue to use dump() format

// Before — always dumps, wrapping strings in quotes
std::string format_as(const json& j) {
  return j.dump();  // "msg" → "\"msg\""
}

// After — unquoted for strings, dump() for everything else
std::string format_as(const json& j) {
  return j.is_string() ? j.get<std::string>() : j.dump();  // "msg" → "msg"
}