Function calculate_cost_optional_split 

pub fn calculate_cost_optional_split(
    model: &str,
    input_tokens: u64,
    output_tokens: u64,
    cache_read: u64,
    cache_creation: u64,
    cache_creation_5m: Option<u64>,
    cache_creation_1h: Option<u64>,
) -> f64

Split-aware cost for callers holding an aggregate plus an optional TTL split (the shape of SessionEvent::Usage). When either split bucket is present, preserve its known count and assign any aggregate remainder to the conservative 1h rate. This is an estimate, not an inferred TTL. A split sum exceeding the aggregate is malformed metadata: retain the larger split sum rather than dropping known writes or lowering the request’s price tier. Arithmetic saturates. When both are None, TTL is explicitly unknown and the existing aggregate-at-5m estimate is retained (potentially underbilling).