azd support for voice agent public preview

[v1212]

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

• feat(agents): support declarative prompt-voice agents #9364
• Use unified API for prompt voice agents #9634
• Support advanced prompt voice agent settings #9655
• Add hosted voice agent support #9697
• Support prompt voice telephony bindings #9804

Notes

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