Function validate_light_client_state_update_certificate 

pub async fn validate_light_client_state_update_certificate<TYPES: NodeType>(
    state_cert: &LightClientStateUpdateCertificateV2<TYPES>,
    membership_coordinator: &EpochMembershipCoordinator<TYPES>,
    upgrade_lock: &UpgradeLock<TYPES>,
) -> Result<()>

Validates a light client state update certificate:

• every signer is in the voting stake table for the cert’s epoch
• each signature is valid (LCV2 always; LCV3 once post-DrbAndHeaderUpgrade)
• the accumulated stake of signers meets the success threshold