feat: add MiniMax provider support

[octo-patch]

Summary

• Add MiniMax chat model provider using OpenAI-compatible interface
• Register MiniMax-M2.7 (default) and MiniMax-M2.7-highspeed models
• Add MINIMAX_API_KEY environment variable support with MINIMAX_BASE_URL override
• Add unit and integration tests for MiniMax provider routing

Changes

rust/crates/api/src/providers/openai_compat.rs

• Added DEFAULT_MINIMAX_BASE_URL = "https://api.minimax.io/v1"
• Added MINIMAX_ENV_VARS constant
• Added OpenAiCompatConfig::minimax() constructor

rust/crates/api/src/providers/mod.rs

• Added ProviderKind::MiniMax variant
• Registered MiniMax-M2.7 and MiniMax-M2.7-highspeed in MODEL_REGISTRY
• Updated metadata_for_model to handle minimax- prefix models
• Updated detect_provider_kind to check MINIMAX_API_KEY

rust/crates/api/src/client.rs

• Added ProviderClient::MiniMax(OpenAiCompatClient) variant
• Added read_minimax_base_url() helper
• Updated all match arms to handle MiniMax

rust/crates/api/tests/provider_client_integration.rs

• Added tests: model routing, missing credentials, base URL override

Usage

export MINIMAX_API_KEY=your_api_key
claudecode --model MiniMax-M2.7

API Reference

• Chat (OpenAI Compatible): https://platform.minimax.io/docs/api-reference/text-openai-api