feat: add MiniMax as LLM provider with Guardian threat protection

[octo-patch]

Summary

Add MiniMax as a first-class provider integration for Guardian SDK. MiniMax offers powerful LLM models (M2.7, M2.5) through an OpenAI-compatible API at https://api.minimax.io/v1. This provider wraps MiniMax-configured OpenAI clients with the same multi-layer threat detection pipeline used for OpenAI and Anthropic.

Changes

• New provider: ethicore_guardian/providers/minimax_provider.py — MiniMaxProvider, ProtectedMiniMaxClient, ProtectedChat, ProtectedCompletions, and create_protected_minimax_client() convenience factory
• Auto-detection: Updated get_provider_for_client() in base_provider.py to detect MiniMax clients by checking base_url for 'minimax'
• Dependencies: Added minimax optional dependency group in pyproject.toml (uses openai>=1.0.0)
• Documentation: Added MiniMax provider example and install instructions to README
• Tests: 30 tests in tests/test_minimax.py — 22 unit tests + 5 integration tests + 3 constant tests

Supported Models

Model	Context
MiniMax-M2.7	1M tokens
MiniMax-M2.7-highspeed	1M tokens (fast)
MiniMax-M2.5	204K tokens
MiniMax-M2.5-highspeed	204K tokens (fast)

Usage

import openai
from ethicore_guardian import Guardian, GuardianConfig
from ethicore_guardian.providers.minimax_provider import MiniMaxProvider

guardian = Guardian(config=GuardianConfig(api_key="my-app"))

minimax_client = openai.OpenAI(
    api_key="your-minimax-api-key",
    base_url="https://api.minimax.io/v1",
)

provider = MiniMaxProvider(guardian)
client = provider.wrap_client(minimax_client)

response = client.chat.completions.create(
    model="MiniMax-M2.7",
    messages=[{"role": "user", "content": user_input}]
)

Test plan

• All 30 unit + integration tests pass with mocked Guardian
• Verify no regressions in existing OpenAI/Anthropic providers
• Manual smoke test with real MiniMax API key (optional)