Function resolve 

pub fn resolve(
    model: &str,
    price_input_per_mtok: Option<f64>,
    price_output_per_mtok: Option<f64>,
) -> Option<ModelPrice>

Resolve the price to bill model at: the explicit config override when BOTH halves are set, else the built-in table, else None.

Both override halves are required together on purpose — an input price with no output price would silently bill completions at zero.