Add MiniMax provider

[octo-patch]

Reason: Add MiniMax as a built-in LLM provider so the executable provider registry covers MiniMax-M3 and MiniMax-M2.7 through the documented global and China OpenAI-compatible endpoints.

Changes

This adds a minimax entry to the built-in provider registry, following the existing OpenAI-compatible provider pattern used by other cloud vendors.

• src/lib/providers/index.ts: register a minimax provider with models MiniMax-M3 and MiniMax-M2.7, requiring an API key and defaulting to the global OpenAI-compatible endpoint https://api.minimax.io/v1. The handler reuses streamOpenAiChat and lets session.apiEndpoint override the default, so the China endpoint (https://api.minimaxi.com/v1) or the Anthropic-compatible endpoint (https://api.minimax.io/anthropic) can be configured.
• src/types/provider.ts: add minimax to the ProviderType union.
• src/lib/server/provider-health.ts: add minimax to OPENAI_COMPATIBLE_DEFAULTS with the global endpoint.
• src/app/api/setup/check-provider/route.ts: route minimax setup checks through the OpenAI-compatible validation branch.
• src/lib/setup-defaults.ts: add a MiniMax entry to SETUP_PROVIDERS with the global endpoint as default and an overridable Base URL (to support the China regional endpoint).
• src/lib/server/provider-health.test.ts: assert minimax is present in OPENAI_COMPATIBLE_DEFAULTS.

Checks

• tsx --test src/lib/providers/provider-models.test.ts (provider registry sanity) — pass
• tsx --test src/lib/server/provider-health.test.ts — pass
• tsx --test src/app/api/setup/check-provider/route.test.ts — pass
• tsx --test src/lib/setup-defaults.test.ts — pass
• eslint on changed files — no new errors
• tsc --noEmit — no new errors in changed files