Cross-tenant subscription disclosure in AshGraphql authorizes notifications in memory without a tenant-scoped read

Incorrect Authorization vulnerability in ash-project ash_graphql allows an authenticated subscriber in one tenant to receive another tenant's records over GraphQL subscriptions.

The subscription resolver in AshGraphql.Graphql.Resolver authorizes each notification payload in memory: its fast path calls Ash.can/3 with run_queries?: false, which evaluates the read policy filter against the in-memory record via Ash.Expr.eval/2 and never issues a query. Ash applies multitenancy at query-build and data-layer-prefix time, not inside query.filter, so the evaluated policy carries no tenant condition and a tenant-B notification routed to a tenant-A subscriber is emitted whenever the policy filter is true. The single-notification clause has no tenant guard at all, and the batched clause checks only the head of the notification list, so non-head entries authorize purely in memory. A tenant-scoped read is reached only when filter evaluation fails.

This issue affects ash_graphql: from 1.4.0 before 1.11.0.

The application exposes ash_graphql subscriptions on a multitenant resource whose read policy passes on in-memory filter evaluation alone, and where a notification created in one tenant can reach a subscriber scoped to another tenant.