Function digest_pm_settlement_inputs 

fn digest_pm_settlement_inputs(
    event_key: &PmSettlementEventKey,
    obligation: &PmSettlementObligation,
    liquid_usdc: Decimal,
    pm_facts: Option<&PmAccountSettlementFacts>,
    pool_snapshot: Option<&PmSettlementPoolSnapshot>,
    policy_version: u32,
    unavailable_reason: Option<&str>,
) -> Result<String, String>