TROVE-2024-004: With full vanguards, some circuits have length 3 rather than the intended 4

Old title: If there are no STUB+ circuits, repurpose the STUB ones

This is about fixing this TODO from circmgr:

        match (circuit.kind, kind) {
            (HsCircStubKind::Stub, HsCircStubKind::Extended) => {
                // TODO HS-VANGUARDS: if full vanguards are enabled and the circuit we got is STUB,
                // we need to extend it by another hop to make it STUB+ before returning it