feat(ai): add MiniMax provider for global and China Anthropic Messages endpoints

[octo-patch]

Reason: Add a validated MiniMax provider preset for the global and China Anthropic Messages endpoints and current MiniMax text models.

Changes

• Add MiniMaxProvider (src/services/ai/providers/minimax.ts) that reuses the Anthropic Messages request/response handling and overrides the endpoint resolution and session provider tag.
• MiniMax exposes an Anthropic Messages-compatible endpoint at <base>/anthropic/v1/messages authenticated with the x-api-key header, on both the global host (https://api.minimax.io) and the China host (https://api.minimaxi.com). The provider normalizes a configured memoryApiUrl base URL so it can be supplied with or without a trailing /anthropic or /anthropic/v1 suffix, then targets /anthropic/v1/messages and keeps the x-api-key + anthropic-version headers from the shared base.
• Refactor AnthropicMessagesProvider to expose sessionProviderTag() and resolveEndpoint() hooks (plus diagnostic log-label hooks) so an Anthropic-compatible gateway records its own provider tag and request URL while sharing the message/tool-call flow; Anthropic behavior is unchanged.
• Wire the "minimax" provider type through AIProviderFactory.createProvider / getSupportedProviders, the AIProviderType union (session-types.ts and types/index.ts), and the memoryProvider config types in config.ts.
• Document the minimax provider mode and the global/China base URLs in the config template and README.

Unlike the closed prior attempt, this adds a source-level provider for both the global and China endpoints, uses the current /anthropic/v1/messages endpoint (not a /anthropic/v1 alias on a single CN host), keeps the current x-api-key auth scheme, and includes tests.

Tests

• bun test tests/minimax-provider.test.ts tests/anthropic-provider.test.ts tests/ai-provider-config.test.ts tests/openai-chat-completion-provider.test.ts tests/config.test.ts tests/config-resolution.test.ts — all passing.
• bunx tsc --noEmit — clean.
• prettier --check on changed files — clean.

New tests/minimax-provider.test.ts covers endpoint resolution for the global host, the China host, base URLs that already include /anthropic or /anthropic/v1, trailing-slash normalization, the missing-URL guard, x-api-key + anthropic-version header emission, request body model/max_tokens, tool-use extraction from an Anthropic-format response, and factory wiring.