fix(models): reuse MiniMax key for CN selections and add MiniMax-M3 to cold-start catalog

[octo-patch]

Reason: CN model selections could not reuse the MiniMax API key, and the cold-start catalog was missing MiniMax-M3.

Changes

• CN auth normalization (lib/public/js/lib/model-config.js): getAuthProviderFromModelProvider now maps the minimax-cn model provider to the minimax auth provider, alongside the existing openai-codex, volcengine-plan, and byteplus-plan mappings. Previously minimax-cn was returned unchanged, so credential fields defined only under minimax were never surfaced for CN selections and MINIMAX_API_KEY could not be reused.
• Cold-start catalog (lib/server/model-catalog-bootstrap.json): added MiniMax-M3 entries for both the minimax and minimax-cn providers, matching the existing regional pattern used for MiniMax-M2.7. The bundled catalog previously exposed the CN provider but omitted MiniMax-M3 on cold start.
• Tests (tests/frontend/model-config.test.js): assert that minimax-cn normalizes to minimax and that the visible AI field keys for minimax-cn include MINIMAX_API_KEY.

Checks

• npx vitest run tests/frontend/model-config.test.js tests/server/model-catalog-cache.test.js — 16 passed.