By Dave Graham, John Revill and Oliver Hirt ZURICH (Reuters) -Writing off 16.5 billion Swiss francs ($20.53 billion) in Credit Suisse bonds was unlawful, a Swiss court ruled on Tuesday, boosting bondholders' hopes of recouping losses and raising fresh questions about how authorities handled the bank's rescue. Swiss market regulator FINMA's decision in March 2023 to wipe out Credit Suisse's Additional Tier 1 (AT1) bonds during the state-engineered takeover by its rival UBS triggered an investor backlash a ...