Function decode_qos_override_value 

pub fn decode_qos_override_value(
    policy: u8,
    value: u32,
) -> Option<QoSOverrideValue>

Decode a (policy, value) code pair. None for an unrecognised policy or an out-of-range enum value.

Split out from decode_qos_override for the FFI paths: they have already matched the topic against a *const c_char, so they need the VALUE without a &'static str to build a whole QoSOverride around.