Make model_obj/provider_fee required (still nullable) on
adjust_payment_for_tokens, get_x_cashu_cost and the private pricing
helpers so a call site that fails to thread the served candidate is a
type error instead of a silent fallback to alias-map re-derivation.
calculate_cost keeps its defaults as the one documented fallback seam.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>