Fix #5333: Preserve vehicle variant on setElementModel

[iManGaaX]

Summary

setElementModel no longer drops the vehicle variant

• Server: CVehicle::SetModel keeps the current variant instead of randomizing it; a new one is only picked when the variation cannot be used on the new model (Slamvan/Caddy/Cropduster/NRG-500/BF-400 need a specific extra), using the new VehicleManager::IsVariationValidForModel

• Client: model changes coming from the server are applied as-is (no more re-randomizing in SetModelBlocking), and client-side setElementModel follows the same rule; servers that do not send the variants keep the old behaviour

• If on(Client)ElementModelChange cancels the change, the original variant is restored

Server and client now report the same getVehicleVariant after a model change

Closes #5333.

Checklist

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