azd support for voice agent public preview

## Summary
Track the azd work needed to make Voice Agent support available for public preview after the remaining dependency PRs land.

## Scope
- Remove the `AZD_AI_AGENT_ENABLE_PROMPT_VOICE` opt-in gate.
- Surface prompt voice and hosted voice capabilities in command help and extension metadata.
- Ensure `azd ai agent` command listings show the public-preview command surface.
- Make the five Voice Agent PR capabilities directly available like the rest of `azd ai agent`.

## Dependency PRs
- #9364
- #9634
- #9655
- #9697
- #9804

## Notes
This should be completed after #9697 and #9804 merge so the public-preview PR can be rebased cleanly onto `main`.