Add MiniMax provider preset with global/CN endpoints

[octo-patch]

Reason: Add MiniMax as a configurable LLM provider with global and CN regional endpoints.

Changes:

• configs/oai_config.py: register a minimax provider preset in get_api_config() (model MiniMax-M3, key MINIMAX_API_KEY, base URL defaulting to the global endpoint https://api.minimax.io/v1) and add minimax to DEFAULT_PROVIDER_PRIORITY so it participates in provider-priority fallback. The CN endpoint (https://api.minimaxi.com/v1) is documented inline and selectable via MINIMAX_BASE_URL.
• configs/mode_config.py: add minimax to the --api-type CLI choices and to the BackendConfig.api_type docstring.
• configs/env.example: add MINIMAX_API_KEY / MINIMAX_MODEL / MINIMAX_BASE_URL entries (defaulting to the global endpoint) and update the DEFAULT_API_PROVIDER comment to list minimax.
• USAGE.md: document the MiniMax configuration block (global/CN endpoints) and the minimax default-provider option.

Checks performed:

• python3 -m py_compile configs/oai_config.py configs/mode_config.py (syntax OK).
• Import test: minimax is present in DEFAULT_PROVIDER_PRIORITY and get_api_config(), and get_llm_config(api_type='minimax') resolves model MiniMax-M3 with base URL https://api.minimax.io/v1.
• Provider-priority fallback test (clean env, only MINIMAX_API_KEY set): validate_and_get_fallback_config() resolves to minimax.
• create_argument_parser() exposes minimax in the --api-type choices.