fix(sse): add the missing minimax-music dispatch to music generation

[octo-patch]

Reason: registered MiniMax music models were impossible to call because the music handler had no branch for their provider format.

Problem

MUSIC_PROVIDERS.minimax declares format: "minimax-music" and its models are published by GET /v1/music/generations and by the specialty model catalog, but handleMusicGeneration never grew a branch for that format. Every minimax/* music request fell through the whole dispatch chain to the final guard and came back as:

400 Unsupported music format: minimax-music

So the models were advertised while being unreachable, and no open or merged PR adds the missing handler.

Change

• open-sse/handlers/mediaGeneration/minimaxMusic.ts (new) — the minimax-music handler. The upstream operation is a single synchronous POST, so the handler:
  • sends model, prompt, lyrics, output_format, audio_setting, lyrics_optimizer, is_instrumental, audio_url, audio_base64 and cover_feature_id, forwarding only the fields the caller actually supplied (instrumental is accepted as the alias the route already uses);
  • pins stream: false — this route answers with a single JSON payload, and streaming responses are restricted to the hex output format anyway;
  • supports both output formats: url (a link that expires after 24h) is returned as { url, format }, hex is decoded and returned as { b64_json, format }, rejecting malformed hex instead of letting Buffer drop bad nibbles silently;
  • restricts audio_setting.format to the mp3 / wav / pcm containers the endpoint accepts and reports the resolved container back to the caller;
  • checks the base_resp envelope (a non-zero status_code also arrives on HTTP 200) and reads the audio from data.audio only once data.status says the generation finished — an unfinished generation is reported back rather than polled, because this operation has no task id and no query endpoint;
  • guards the missing-credential case locally (401) instead of sending an unauthenticated request upstream, and records every outcome through saveCallLog like the other media handlers.
• open-sse/handlers/musicGeneration.ts — dispatch minimax-music to that handler, next to the other provider-format branches.
• open-sse/config/musicRegistry.ts — add the generation and cover model ids the entry was missing, declare the regional endpoint (regionalBaseUrl), and drop the query URL that does not exist for this operation. A connection reaches the regional endpoint through the existing per-connection providerSpecificData.baseUrl override; that endpoint is also the only one that accepts the aigc_watermark request field, so the field is forwarded there and dropped otherwise.
• tests/unit/minimax-music-generation.test.ts (new) — 9 regression tests: registry shape, dispatch + URL normalization + request-field mapping, hex → base64, cover inputs, regional endpoint + regional-only field, base_resp failure on HTTP 200, unfinished generation, completed-without-audio, upstream HTTP failure, and the missing-credential guard.
• changelog.d/fixes/minimax-music-generation-dispatch.md — changelog fragment.

No behavior changes for any other music provider format: the new branch is additive and sits before the unsupported-format guard.

Checks

Run in this environment:

• node --test tests/unit/minimax-music-generation.test.ts — 9/9 pass. Executed with Node 24's native TypeScript execution; the usage-log module (@/lib/usageDb) was stubbed because the workspace dependencies could not be installed here, so npm run test:unit and the ESLint / typecheck gates still need to run in CI.
• node scripts/check/check-changelog-integrity.mjs — OK.
• node scripts/check/check-file-size.mjs — OK (no baseline change needed: the new files are well under the cap and musicGeneration.ts is not frozen).

[diegosouzapw]

Reviewed and validated: checked out the branch, ran tests/unit/minimax-music-generation.test.ts — 9/9 pass — and confirmed ESLint/typecheck show no new errors in the touched files (the two failures observed are pre-existing base-tip issues unrelated to this change: one already allowlisted in eslint-suppressions.json, the other in an unrelated compression-engine file this PR never touches). The minimax-music dispatch gap is real (models registered/advertised via /v1/models but every request 400'd with Unsupported music format), the fix is additive and isolated, and it follows existing codebase conventions (providerSpecificData.baseUrl override, sanitizeErrorMessage, saveCallLog). Looks good to merge.