feat(model_apis): add MiniMax Cloud API as text generation provider

[octo-patch]

Summary

Add MiniMax as the 4th cloud text-generation API provider (alongside OpenAI, Cohere, and Claude).

xTuring already has excellent MiniMax M2 local fine-tuning support (PR #307) — this PR complements it by adding cloud API access to MiniMax latest models (M2.7 / M2.7-highspeed) for self-instruct data generation and text generation workflows.

Changes

File	Description
src/xturing/model_apis/minimax.py	MiniMaxTextGenerationAPI — OpenAI-compat client at api.minimax.io/v1, temperature clamping, retry with exponential backoff. Convenience classes MiniMaxM27 and MiniMaxM27HighSpeed.
src/xturing/model_apis/init.py	Register minimax, minimax_m2_7, minimax_m2_7_highspeed in BaseApi registry
tests/xturing/model_apis/test_minimax_api.py	18 unit tests + 3 integration tests (skip when MINIMAX_API_KEY not set)
README.md	Cloud API table with all 4 providers, usage example, roadmap update

Usage

from xturing.model_apis import MiniMaxM27

api = MiniMaxM27(api_key="your-minimax-api-key")
results = api.generate_text(
    prompts=["Explain quantum computing for beginners."],
    max_tokens=256,
    temperature=0.7,
)
print(results[0]["response"]["choices"][0]["text"])

Test plan

• 18 unit tests pass
• 3 integration tests pass with live MINIMAX_API_KEY
• Existing Claude API tests still pass (pre-existing failures unrelated)
• No new dependencies — reuses existing openai SDK