Currently for pipeline commands, the PQS validation occurs outside of the aggregation_request_helper::validate() function in 2 places:
1. PipelineCommand::parseForExplain()
2. ClusterPipelineCommand::parseForExplain()

We should be able to centralize these checks into aggergation_request_helper::validate() in this form:

void validatePipeline(..., bool isClusterPipeline) {
    if (isClusterPipeline) {
        clusterPQSValidation();
    } else {
        regularPipelinePQSValidation();
    }
    ... // rest of the validation code
}

This will also require identifying the other users of aggregation_request_helper::parseFromBSON() which calls validate() and ensure we do the appropriate handling in those cases.