Fix type of a computed prop in query builder

[diksipav]

Schema

module default {
  type User {
    ...
    multi movies: Movie;
    multi shows: Show;
    multi documentaries: Documentary;

    multi watching_list := ( 
      select .movies union .shows
      order by .year
    );

    multi all_media := (
       select .movies union .shows union .documentaries
    )
  }

  abstract type Content {
    required year: int16;
    required title: str;
  }

  type Movie extending Content {
   required plot: str;
  }

  type Show extending Content {
   required seasons: int16;
  }

  type Documentary {
    required title: str;
    required plot: str;
  }
};

In the test below type of plot prop should be string | null.

const q = e.select(e.User, () => ({
      all_media: (all_media) => ({
        title: true,
        ...e.is(e.Content, { year: true }),
        ...e.is(e.Show, {
          seasons: true,
        }),
        plot: e.op(
          all_media.is(e.Movie).plot,
          "??",
          all_media.is(e.Documentary).plot,
        ),
      }),
    }));

    type qT = $infer<typeof q>;

    tc.assert<
      tc.IsExact<
        qT,
        {
          all_media: (
            | {
                title: string;
                plot: string;
                __typename: "default::Documentary";
              }
            | {
                title: string;
                plot: string;
                year: number;
                __typename: "default::Movie";
              }
            | {
                title: string;
                plot: string;
                year: number;
                seasons: number;
                __typename: "default::Show";
              }
          )[];
        }[]
      >
    >(true);

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