Distinct-query aggregate drops the tenant schema prefix, leaking across tenants in AshSql

Incorrect Authorization vulnerability in ash-project ash_sql allows a caller in a schema-based multitenant application to receive aggregate values computed from another tenant's rows.

When an aggregate is computed over a distinct query, AshSql.AggregateQuery.add_single_aggs/5 rebuilds the outer query from query.from.source alone, which is only the {table, schema} tuple and does not carry query.prefix or query.from.prefix. For strategy(:context) multitenancy those hold the tenant schema, so the rebuilt outer query reads the repo's default schema while the inner correlated subquery still reads the tenant schema, and the two are joined only on primary key. The aggregate, and any relationship join added off the prefix-less binding, is then computed against the wrong tenant's rows. The neighbouring limit and exists branches instead wrap the query with subquery/1, which preserves the prefix.

This issue affects ash_sql: from 0.1.0 before 0.7.1.

An application must use AshPostgres schema-based (strategy(:context)) multitenancy and compute an aggregate over a distinct query (for example an aggregate whose query filters across a to-many relationship).