Add MiniMax music cover endpoint

[octo-patch]

Reason: Add MiniMax music cover support with regional endpoints and validated cover inputs.

This change:

• adds a MiniMax music cover API route and typed client for the global and China endpoints
• supports the music-cover and music-cover-free models, URL or base64 audio input, cover features, output controls, and audio settings
• validates cover input selection, size, streaming format, audio format, and regional watermark usage
• handles MiniMax transport, base response, cover status, and audio output responses

Checks:

• npx --yes tsx@4.20.6 --test src/lib/MiniMaxMusicApi.test.ts
• ./node_modules/.bin/tsc --noEmit --incremental false
• ./node_modules/.bin/next lint --file src/lib/MiniMaxMusicApi.ts --file src/lib/MiniMaxMusicApi.test.ts --file src/app/api/minimax_music_cover/route.ts
• npm run build

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