By Amanda Cooper LONDON, Dec 11 (Reuters) - The cost of insuring Oracle's debt against the risk of default has shot up after its latest earnings reignited worries about how much the broader corporate sector is spending on AI and the borrowing surge to fund it. A growing debt pile, at just ​over $100 billion, means Oracle has become a bellwether for sentiment towards AI as concern about a bubble in the sector grows with surging tech shares. Trading in ‌Oracle credit default swaps (CDS), which has explod ...