Fail-closed governance backend failures are semantically scope-level
unavailability, yet GovernanceBackendError sits directly under
PolyGatewayError, so callers writing only `except GatewayUnavailableError`
drop them into the catch-all bucket and burn their failure budget on a
fault that a restart would clear.
The design reparents it under GatewayUnavailableError with a new
governance_backend_down reason, splits the two "unknown source" sites into
a separate assembly-defect error so a misconfiguration still reaches the
dead letter queue, and picks a non-zero retry_after_s to avoid a
zero-delay retry storm against a backend that is already down.
