AshPostgres ignores attribute multitenancy on the join resource when loading many_to_many and through relationships, returning records related through other tenants' rows

Incorrect Authorization vulnerability in ash-project ash_postgres allows a user to load destination records linked to a source record only through another tenant's join rows.

When a many_to_many relationship, or a through relationship, is loaded with a lateral join (for example with a per-record limit or sort), AshPostgres.DataLayer builds the query for the join resource, and for each resource along a through path, in lateral_join_query/3 and through_step_queries/4 without running that resource's read action, which is where attribute multitenancy is applied. When those resources use strategy :attribute multitenancy, rows belonging to other tenants were joined, so the load returned destination records that the caller's tenant is not related to.

This issue affects ash_postgres: from 0.38.0 before 2.15.2.

Reachable when the join resource of a many_to_many relationship, or a resource along a through path, uses attribute multitenancy and holds rows from several tenants, and the relationship is loaded with a lateral join (for example with a limit, offset or sort on the relationship).