Feature Request: Add MiniMax as native AI provider

MiniMax is a growing AI provider with an OpenAI-compatible API. Adding native support would help
users in Asia and beyond access MiniMax's models through ruflo.

## Current Situation
- MiniMax has OpenAI-compatible endpoints at `https://api.minimax.chat/v1`
- Users can add it as a custom provider via config file
- The `config providers` CLI command only displays built-in providers, not custom ones

## Requested Features
1. **Native MiniMax provider** with built-in model list (MiniMax-01, etc.)
2. **Custom provider visibility** in `config providers` command
3. **Provider validation** - test connectivity when adding custom providers

## API Details
- Base URL: `https://api.minimax.chat/v1`
- API Key format: `sk-cp-233-...`
- Documentation: https://platform.minimaxi.com/

Regards,

Paul Gooch

Start with the existing `config providers` CLI command and the custom-provider configuration path, then review the MiniMax API documentation at platform.minimaxi.com. Done means MiniMax appears as a native provider with its model list, custom providers appear in the command output, and adding one validates connectivity.

Written by the indexing model from the issue text.