feat: upgrade MiniMax default model to M3

[octo-patch]

Summary

Upgrade MiniMax default model from M2.5/M2.7 to M3 across the Go backend, frontend constants, and docs. M3 is the latest MiniMax flagship: 512K context window, up to 128K output, and image input support.

M2.7 / M2.7-highspeed remain selectable by users via the existing custom-model field; only the default is changed.

Changes

Backend (Go)

• mcp/providers.go: DefaultMiniMaxModel MiniMax-M2.7 → MiniMax-M3
• mcp/provider/minimax.go: DefaultMiniMaxModel MiniMax-M2.7 → MiniMax-M3
• api/handler_ai_model.go: supported-models list default MiniMax-M2.7 → MiniMax-M3
• agent/agent.go: resolveModelRuntimeConfig minimax entry — model MiniMax-M2.5 → MiniMax-M3, URL api.minimax.chat → api.minimax.io (correct endpoint)
• agent/model_provider_catalog.go: supportedModelProviders minimax DefaultModel MiniMax-M2.5 → MiniMax-M3
• store/strategy.go: contextLimitMinimax 1_000_000 (1M) → 512_000 (512K) to match M3 spec

Frontend (TypeScript)

• web/src/components/trader/model-constants.ts: AI_PROVIDER_CONFIG.minimax.defaultModel MiniMax-M2.7 → MiniMax-M3

Docs

• docs/token-estimation.zh-CN.md: MiniMax row context window 1,000,000 / 800,000 → 512,000 / 409,600

Why M3?

MiniMax-M3 is the latest flagship from MiniMax, with a 512K context window, up to 128K output tokens, and (in OpenAI-compatible mode) image-input support. The OpenAI-compatible API endpoint (https://api.minimax.io/v1) and auth scheme are unchanged, so no client-side wiring needs to be touched.

Compatibility

• Users who hard-coded MiniMax-M2.7 (or earlier) via the custom-model UI field are unaffected — the change only updates the default model.
• Older models (MiniMax-M2.5/M2.1/M2/M1) were never on the picker list in dev — only defaults were updated.

Test Plan

• mcp / agent / api / store compile cleanly (go build ./...)
• No references to deprecated api.minimax.chat remain in code
• No 1_000_000 1M context literal remains for minimax
• Frontend AI_PROVIDER_CONFIG.minimax.defaultModel is MiniMax-M3

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