fix: align music CLI and SDK with the current API contract

[NianJiuZst]

Summary

• accept the official music-2.6-free and music-cover-free models in CLI and SDK validation
• normalize the SDK's instrumental convenience alias to the official is_instrumental field without sending SDK-only fields to the API
• require a prompt for instrumental and lyrics-optimizer requests, matching the API contract
• decode SSE hex audio in SDK streaming instead of yielding raw protocol frames
• centralize music model lists and update CLI/SDK examples

Root cause

The model allowlists lagged behind the current API, the SDK validated a different field from its documented instrumental alias, and its streaming path bypassed the SSE/hex decoding already used by the CLI.

Impact

Free-tier users can use both supported music models through the CLI. SDK instrumental examples now produce an API-compatible request, and streaming consumers receive audio bytes rather than SSE text.

Official contract: https://platform.minimax.io/docs/api-reference/music-generation

Related to #182.

Validation

• bun test — 384 passed
• targeted Music SDK tests — 16 passed
• bun run typecheck
• bun run build:dev
• ESLint on all changed TypeScript files
• git diff --check

[NianJiuZst]

Reviewed by GPT-5.6 Sol Ultra — approved, with no issues identified.