[upstream #17033] 17033: Tenantless account erasure runs ID-keyed cleanup without a tenant predicate #357

Found while reviewing LibreChat-AI#17033 (Codex finding LibreChat-AI#17033 (comment)). When a user document has no tenantId (legacy, pre-tenancy), account erasure runs without a tenant filter: the CLI uses runAsSystem, and the self-service request has no tenant context in non-strict mode. ID-keyed cleanup (deleteUserAgents handoff edges and favorites by agent id, processDeleteRequest metadata and agent references by file_id) can then touch a record in a real tenant that reuses the same id, since agent ids are unique only by { id, tenantId } and file_id is not unique. It requires a deliberate id collision across tenants and reproduces on dev for self-service deletion. Use explicit no-tenant predicates (tenantId: { $exists: false }) for these operations when the deleted user has no tenant.