reject and modify parameters should not be mutually exclusive

[oseoin]

AI Gateway core will only rely on the modify parameter when validating if processors can be run concurrently, as a result there is no need to block reject when modify is enabled.

Removing this rule allows processor developers to design a processor to either modify or reject depending on how they classify the input while continuing to allow the AI Gateway administrator to control what they are allowed to do for a given request.