docs: public SafeUrl toSafeUrl() {
      Preconditions.checkState(
          getContentKind() == ContentKind.URI,
          "toSafeUrl() only valid for SanitizedContent of kind URI, is: %s",
          getContentKind());
      return UncheckedConversions.safeUrlFromStringKnownToSatisfyTypeContract(getContent());
    }