PR #9463 switched minimax/minimax-cn from claude to openai format so images
work. The stream-readiness bump for Claude-format replicas is keyed off the
registry's format field (single source of truth), so minimax legitimately
falls out of that group now. Swap the "Claude-format replica" test fixtures
to agentrouter (still format: "claude") and add explicit coverage that
minimax no longer gets the claude_format_heavy_reasoning bump.
f52b60d846