Thrown by the {@see \LaravelUi5\Sdk\Partners\Models\PartnerRelationship} write
guard when a save would produce a second active, time-overlapping primary
relationship for the same partner and relationship type.
At most one active primary per (partner_id, relationship_type_id) is the
invariant the org-scoping family (OrgPartnerResolver / #[ScopedToOrgActors])
relies on: two overlapping active primaries would scope an actor to several
orgs at once and leak rows. This is a developer/data error (the caller must
end or demote the existing primary first), not a user-facing authorization
problem — hence a loud RuntimeException, transport-agnostic so it surfaces in
HTTP, console (ui5:sync, seeders) and queue contexts alike.
See https://laravelui5.com/sdk/partners/overview