test: cover vault grant options

[pjpjq]

Summary

• include is_grantable in Vault relation and function privilege snapshots
• cover the standard, Multigres, and Multigres OrioleDB test variants
• assert that only the postgres Vault grants configured with WITH GRANT OPTION are grantable

Validation

• ran nix/tests/sql/vault.sql against local supabase/postgres:17.6.1.107
• compared all 37 ACL result rows with nix/tests/expected/vault.out
• verified the three Vault SQL variants and their ACL expected-output blocks stay identical
• full Nix/pg_regress suite not run because nix and pg_regress are not installed locally

Partial progress on #1570