feat: add MiniMax LLM provider support

[octo-patch]

Summary

Add MiniMax as a new LLM provider, following the existing provider architecture pattern (similar to DeepSeek, Kimi, GLM, Qwen).

MiniMax offers an OpenAI-compatible API with high-performance models:

• MiniMax-M2.5 — 204K context window, suitable for general dialogue, code generation, and complex reasoning
• MiniMax-M2.5-highspeed — Optimized version with faster inference, same 204K context

Changes

New files

• backend/pkg/providers/minimax/minimax.go — Provider implementation using langchaingo OpenAI client
• backend/pkg/providers/minimax/config.yml — Model configurations for all option types (simple, primary_agent, assistant, generator, coder, etc.)
• backend/pkg/providers/minimax/models.yml — Model definitions with descriptions and pricing

Modified files

• backend/pkg/providers/provider/provider.go — Add ProviderMiniMax type constant and DefaultProviderNameMiniMax
• backend/pkg/config/config.go — Add MINIMAX_API_KEY, MINIMAX_SERVER_URL, MINIMAX_PROVIDER environment variable config fields
• backend/pkg/providers/providers.go — Register MiniMax in provider factory (import, default config, instantiation, NewProvider switch case)

Configuration

Set these environment variables to enable MiniMax:

MINIMAX_API_KEY=your_api_key_here
# Optional: override default server URL
MINIMAX_SERVER_URL=https://api.minimax.io/v1
# Optional: model prefix for Langfuse logging
MINIMAX_PROVIDER=

Test Plan

• Verify Go build compiles without errors
• Test provider instantiation with valid MiniMax API key
• Verify model selection for different option types (simple, primary_agent, coder, etc.)
• Test streaming and tool calling capabilities
• Verify provider appears in the UI provider list when API key is configured