code: public <T> AsyncMutateInBuilder upsert(String path, T fragment) {
          if (StringUtil.isNullOrEmpty(path)) {
              throw new IllegalArgumentException("Path must not be empty for upsert");
          }
          this.mutationSpecs.add(new MutationSpec(Mutation.DICT_UPSERT, path, fragment));
          return this;
      }
docs: Insert a fragment, replacing the old value if the path exists.
  
  @param path the path where to insert (or replace) a dictionary value.
  @param fragment the new dictionary value to be applied.