Saudi Arabia is celebrating one of the biggest foreign-investment windfalls in its history after netting more than $12 billion by selling off a stake in the oil pipelines that traverse the desert kingdom.But the country may also be facing an uncomfortable reality as a result. As carefully cultivated relationships with firms such as BlackRock Inc. and SoftBank Group Corp. have yet to draw in the desired investment, it’s turning to the jewels of its energy industry to attract new money.