exists/2 predicate silently dropped on limited relationships with a parent() filter in AshSql

Incorrect Authorization vulnerability in ash-project ash_sql allows a caller to bypass a scoping or authorization filter expressed as exists/2 over a relationship that declares both a limit (or from_many?) and a parent(...)-referencing filter or sort.

AshSql.Join.related_query/3 skips the caller-supplied exists predicate for such relationships and delegates it to limit_from_many/5. When the relationship's own filter or sort references parent(...), limit_from_many/5 takes a branch that drops both the limit and the predicate, emitting a bare correlated EXISTS with no predicate. The check then matches any record that has any related row. Most severely, when the expression backs a policy (for example authorize_if expr(exists(memberships, user_id == ^actor(:id)))), the actor-scoping condition disappears and the policy passes for any actor with any related row.

This issue affects ash_sql: from 0.4.1 before 0.7.1.

An application must use exists/2 (in a filter or a policy) over a relationship that declares both a limit or from_many? and a filter or sort that references parent(...). The predicate supplied to exists/2 is the one that is dropped.