EEF-CVE-2026-78699

rename_tenant returns :ok on a failed rename, enabling cross-tenant access in AshPostgres

## Summary Unchecked Return Value vulnerability in ash-project ash_postgres allows a user who can drive a tenant rename to a name that collides with an existing tenant's schema to have their tenant record repointed at that other tenant's live schema, gaining access to its data. `AshPostgres.MultiTenancy.rename_tenant/3` issues the `ALTER SCHEMA ... RENAME TO ...` with the non-raising `Ecto.Adapters.SQL.query/2`, discards its `{:ok, _} | {:error, _}` result, and unconditionally returns `:ok`. PostgreSQL rejects the rename when the target schema already exists (and on insufficient privilege or lock timeout), but that failure never reaches the caller. The calling `manage_tenant` update action therefore sees success and commits the tenant row with the new name, which is the schema of a different existing tenant, so subsequent reads and writes for that tenant run against the other tenant's data. This issue affects ash_postgres: from 0.25.0 before 2.13.0. ## Configurations An application must run ash_postgres schema-based multitenancy with a user-driven tenant rename (for example a `manage_tenant` update on an editable org slug or subdomain), letting a user choose a new tenant name that collides with an existing tenant's schema.