Add MiniMax preset provider defaults

[octo-patch]

Reason: Add MiniMax as a preset LLM provider with default models and region presets.

Adds MiniMax to the backend provider registry and default model resolver, updates the BYOK profile UI with the new preset provider and MiniMax model choices, and adds targeted tests for the new preset path.

Checks:

• pytest backend/tests/test_unit_schemas.py backend/tests/test_custom_mode_api_key_simple.py backend/tests/test_unit_config_store.py
• npx eslint "app/[locale]/profile/page.tsx"