Fixes#16825. Sessions using MiniMax-M2.7 via minimax-cn showed
estimated_cost_usd=0.0 and cost_status='unknown' because neither
provider had a billing route or pricing entry. Adds official_docs_snapshot
entries ($0.30/M input, $1.20/M output) for both minimax and minimax-cn,
and adds explicit routing in resolve_billing_route so both resolve to
billing_mode='official_docs_snapshot' instead of falling through to 'unknown'.