Tenant-owned records are partitioned by RequestContext::tenant_id. That
affects both task issue references and backend registration names:
- the same issue reference may exist once per tenant,
- the same backend name may exist once per tenant, and
- lookups return only the caller's tenant-owned records.
When tests or application code reuse an external identifier or backend name for
records that are intended to belong to different tenants, a distinct
RequestContext should be used for each tenant.