CVE-2026-35413: Directus: GraphQL Schema SDL Disclosure Setting

When GRAPHQL_INTROSPECTION=false is configured, Directus correctly blocks standard GraphQL introspection queries (__schema, __type). However, the server_specs_graphql resolver on the /graphql/system endpoint returns an equivalent SDL representation of the schema and was not subject to the same restriction. This allowed the introspection control to be bypassed, exposing schema structure (collection names, field names, types, and relationships) to unauthenticated users at the public permission level, and to authenticated users at their permitted permission level.