Add MiniMax provider routing to the PRICE API judge

[octo-patch]

Reason: The PRICE API judge routes to several providers but has no MiniMax mapping, so its MiniMax-M3 / MiniMax-M2.7 judges and MiniMax endpoints cannot be selected.

What changed

The executable PRICE API judge (evaluation/image_gen/PRICE/scripts/eval_imgs.py) already routes judge aliases to per-provider official APIs (with an aggregator fallback) via a set of registration tables, and its OpenAI-compatible chat client and multimodal (text + image) message builder are the shared extension point. This PR registers MiniMax through those same tables, reusing the existing client and message builder without introducing a new code path:

• Added the minimax-m3 and minimax-m2.7 judge aliases to API_JUDGE_MODEL_ALIASES (mapping to MiniMax-M3 and MiniMax-M2.7).
• Mapped both aliases to the minimax provider in JUDGE_ALIAS_PROVIDER.
• Added the minimax default base URL (https://api.minimax.io/v1, the global OpenAI-compatible endpoint) to PROVIDER_DEFAULT_BASE_URL, with an inline note that the mainland China site (https://api.minimaxi.com/v1) can be selected via the existing per-provider base_url override.
• Declared the official model ids for both aliases in JUDGE_ALIAS_OFFICIAL_MODEL_ID.

Because the aliases live in API_JUDGE_MODEL_ALIASES, they are automatically picked up by --model, --all-api-models, and the scores_of_judges.md ordering with no further wiring.

Documentation was updated to match:

• env.example.toml gains a [minimax] table with the global default and the commented mainland China base_url override.
• README.md documents the two new aliases and the global/China endpoint selection.

Checks

• python3 -m py_compile scripts/eval_imgs.py
• python3 -m pytest tests/ (1 passed)
• Ad-hoc verification of the routing functions: alias registration, auto/official route resolution, official vs. aggregator model-id selection, and global-default vs. China-base_url-override endpoint selection for both models.