ESQL: union types use duplicate attribute names #110490
Description
In ES|QL, attribute names (like x,y,z in the query FROM idx | x = 1, y = x + some_field, z = y * 2) are generally unique at any step of the query ("after each pipe").
#110488 tries to enforce this, but fails due to union types adding multiple attributes with the same name if two indices have the same field with different types. We should either make the names unique, or somehow consolidate the two attributes into one.
(Buildscan with failed validation: https://gradle-enterprise.elastic.co/s/foddxnpum3ktc/tests/overview)