Function failover 

pub fn failover(
    req: &SelectionRequest<'_>,
    failed: &CredentialRef,
    evidence: &FailureEvidence,
    candidates: &[AccountCapacity],
) -> FailoverDecision

Decide whether the request may be retried on another seat.

Never replays after output or tool activity, never on an ordinary 429, never for an explicitly requested account, and at most MAX_FAILOVERS times. The failed seat is excluded from the fresh selection.