feat: add MiniMax as LLM provider for Planner & Coder agents

[octo-patch]

Summary

• Add MiniMax as an alternative LLM provider for the Planner & Coder agents
• MiniMax M2.7 model with 1M context window via OpenAI-compatible API (https://api.minimax.io/v1)
• No new dependencies required — reuses the existing openai SDK

Changes

File	Change
src/gpt_request.py	Add request_minimax() and request_minimax_token() functions
src/api_config.json	Add minimax config entry (base_url, api_key, model)
src/agent.py	Register "minimax" in API mapping and argparse choices
README.md	Mention MiniMax in LLM API section
tests/test_minimax_provider.py	16 unit tests + 3 integration tests

Usage

# Set your MiniMax API key in api_config.json or via env var
export MINIMAX_API_KEY=your_key_here

# Run with MiniMax
sh run_agent_single.sh --API minimax --knowledge_point "Linear transformations and matrices"

Test plan

• 16 unit tests pass (config, request functions, agent integration, retry/error handling)
• 3 live integration tests pass against MiniMax API
• Run full video generation pipeline with --API minimax