Adds FALLBACK_PRICING entries plus display names so MiniMax sessions
show up with the right cost and readable labels when users route MiniMax
through providers like OpenCode. Pricing verified against the live
MiniMax paygo page:
MiniMax-M2.7 input $0.3/M output $1.2/M cache-read $0.06/M cache-write $0.375/M
MiniMax-M2.7-highspeed input $0.6/M output $2.4/M cache-read $0.06/M cache-write $0.375/M
