Every provider's voices in one list, for the unified voice picker.

Replaces the three per-provider endpoints above for any caller that wants voices rather than a vendor's voices.

This endpoint never returns 503, and that is deliberate. Its siblings do, because a picker bound to one provider has nothing to show when that provider is down. A union has plenty: a Cartesia outage must not empty a picker holding 1,568 Google voices. So a provider that cannot be reached is named in unavailable and the rest are still served. Do not "fix" this into a 503 for consistency with its neighbours.

An unsupported language is a 422 rather than an empty list, so a typo reads as a mistake instead of as "no voices speak this".