feat(provider): Add MiniMax AI provider integration

- Add minimax to PROVIDERS lists in gui_2.py and app_controller.py
- Add minimax credentials template in ai_client.py
- Implement _list_minimax_models, _classify_minimax_error, _ensure_minimax_client
- Implement _send_minimax with streaming and reasoning support
- Add minimax to send(), list_models(), reset_session(), get_history_bleed_stats()
- Add unit tests in tests/test_minimax_provider.py

src/app_controller.py

@@ -110,7 +110,7 @@ class AppController:
  The headless controller for the Manual Slop application.
  Owns the application state and manages background services.
  """
- PROVIDERS: list[str] = ["gemini", "anthropic", "gemini_cli", "deepseek"]
+ PROVIDERS: list[str] = ["gemini", "anthropic", "gemini_cli", "deepseek", "minimax"]
 
  def __init__(self):
  # Initialize locks first to avoid initialization order issues