Add OpenAI-compatible provider selection for the MiniMax endpoint

[octo-patch]

Reason: Add OpenAI-compatible provider selection so the agent can target the MiniMax global and China region endpoints.

This PR adds provider selection to the OpenAI-compatible client without changing the default behaviour:

• run.py: add --api_provider and --api_base_url arguments and a build_openai_client helper. minimax selects the global endpoint https://api.minimax.io/v1 and minimax_cn selects the China endpoint https://api.minimaxi.com/v1. --api_base_url overrides directly. When neither is set the client keeps the original OpenAI default, so screenshot and text-only modes are unchanged.
• run.sh: document how to run against an OpenAI-compatible provider endpoint.
• README.md: document the two new options.

Checks run:

• python3 -m py_compile run.py
• Isolated unit check of build_openai_client: default OpenAI endpoint preserved; minimax -> https://api.minimax.io/v1, minimax_cn -> https://api.minimaxi.com/v1, --api_base_url override applied.