Function htlc_completion_error 

fn htlc_completion_error(
    err: &NodeError,
    payment_hash: &str,
) -> LightningRpcError

Classifies an ldk-node claim or fail error for the gateway’s completion retry loop.

claim_for_hash and fail_for_hash fail deterministically for an unknown payment hash, a preimage that does not hash to it, or an amount below the registered one. Retrying cannot change any of those, so they are reported as LightningRpcError::HtlcCompletionRejected and the completion state machine records the outcome instead of retrying forever. Everything else, today only a failed store write, is treated as transient: the incoming contract is already funded by the time this runs, so an error this list does not know must keep retrying rather than be recorded as final.