feat(eval): add regional MiniMax metadata

[octo-patch]

Reason: Refresh MiniMax evaluation configuration with selectable regional endpoints and current model metadata.

Summary

• add global and China OpenAI- and Anthropic-compatible endpoint pairs
• select endpoint pairs with MINIMAX_API_REGION while preserving independent overrides
• expose current context, pricing, modality, and thinking metadata for registered models
• document regional selection and cover the configuration with regression tests

Checks

• uv run pytest tests/test_eval_model_config.py -q
• uv run python -m py_compile eval/lib/model_config.py tests/test_eval_model_config.py

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