FleetDispatchException

An assignment policy named a vehicle that had not declared itself available.

Availability is asserted by a vehicle, never inferred by the dispatcher (invariant A6), so a policy that returns a vehicle the dispatcher did not offer it has reached outside what it was given. That is a policy defect rather than a modelling condition, which is why it raises instead of being skipped: a silently dropped proposal would show up much later as a fleet that never quite keeps up.