[Together] Default text-to-speech voice

[hanouticelina]

Comment from @hanouticelina : this PR should fix the widget for text-to-speech with Together

Summary

Together's /v1/audio/speech requires a voice field, but the SDK didn't set one — so any TTS call that didn't pass parameters.voice failed with HTTP 400 "voice is required". This also caused the periodic HF mapping validator to flip hexgrad/Kokoro-82M to status: "error".

Defaults voice to af_alloy (a valid Kokoro voice) only when the target model is Kokoro — the only TTS model currently registered for Together. User-supplied parameters always override. Other model families (Orpheus, Cartesia, …) get no default and continue to surface Together's clear "voice is required" error if the caller omits one.

Behavior matrix

Call	Before	After
Kokoro, no voice	400 "voice is required"	voice: "af_alloy" ✓
Kokoro, parameters: { voice: undefined }	400	voice: "af_alloy" ✓
Kokoro, parameters: { voice: "af_bella" }	af_bella ✓	af_bella ✓ (user override)
Future non-Kokoro, no voice	400	400 "voice is required" (no wrong default)
Future non-Kokoro, parameters: { voice: "tara" }	tara ✓	tara ✓

Test plan

• pnpm --filter @huggingface/inference run check (tsc) — clean
• pnpm --filter @huggingface/inference run lint:check — clean
• pnpm --filter @huggingface/inference run format — clean
• Live against api.together.xyz with hexgrad/Kokoro-82M:
  • no parameters → 108.9 KB WAV (default af_alloy)
  • parameters: {} → 108.5 KB WAV (default af_alloy)
  • parameters: { voice: undefined } → 108.6 KB WAV (default af_alloy)
  • parameters: { voice: "af_bella" } → 92.9 KB WAV (user override)
• Mock-fetch on a synthetic non-Kokoro model:
  • no voice → body.voice absent (no wrong default)
  • voice: "tara" → body.voice: "tara" (user value passes through)
• Reproduced the failure on main for direct comparison

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