feat(model): add MiniMax provider with global and China regions

[octo-patch]

Reason: Add the MiniMax model provider with global and China regions, exposing the current MiniMax-M3 and MiniMax-M2.7 models.

Changes

This adds MiniMax as a first-class built-in model provider, discovered and registered automatically by the existing provider auto-discovery mechanism (no manual registration needed).

• New provider backend/app/core/model/providers/MiniMax.py: a MiniMaxProvider extending the OpenAI-compatible base. It exposes both regional endpoints and both API protocols:
  • Regions: global (https://api.minimax.io) and China (https://api.minimaxi.com).
  • OpenAI-compatible base URLs: https://api.minimax.io/v1 and https://api.minimaxi.com/v1.
  • Anthropic-compatible base URLs: https://api.minimax.io/anthropic and https://api.minimaxi.com/anthropic.
  • A region credential field selects the endpoint, and a protocol_type field (openai | anthropic) selects which compatible API to use; an optional base_url overrides either.
  • Predefined chat models: MiniMax-M3 (1M context, multimodal input, adaptive thinking) and MiniMax-M2.7 (204K context, always-on thinking).
  • Credential validation and model instance creation honor the resolved region/protocol base URL.
• Display order backend/app/services/model_provider_service.py: register minimax in BUILTIN_PROVIDER_ORDER so it sorts with the other built-in providers.
• Tests backend/tests/test_minimax_provider.py: cover provider identity, credential schema, predefined models, region/protocol base-URL resolution for both protocols in both regions, explicit base URL override, unknown-region fallback, and error handling.

The provider is picked up by get_all_provider_instances() / ModelFactory via directory auto-discovery, so no __init__.py registration is required.

Checks

• ruff check on the changed files — passed.
• ruff format --check on the changed files — passed.
• mypy app/core/model/providers/MiniMax.py --ignore-missing-imports — no issues.
• pytest tests/test_minimax_provider.py — 12 passed.

[octo-patch]

Updated the MiniMax provider to use the canonical global_en/cn_zh region identifiers and expose the complete context, pricing, input modality, and thinking metadata for MiniMax-M3 and MiniMax-M2.7. I ran the 12 provider tests, Ruff check/format, and a targeted mypy check.