Add opt-in unified API mode for prompt voice agents

[JianW (v1212)]

Summary

• Adds an opt-in AZURE_VOICE_AGENT_API mode for prompt voice agents so azd can deploy through the unified /agents API while keeping legacy /voice_agents as the default.
• Supports legacy, unified, and unified-flat modes to cover current regional rollout and West US 2 TiP/spec validation.
• Updates unified mode to publish the voice WebSocket endpoint path and to use /agents/{name} for repeat deploys when an agent name already exists in the azd environment.

Why

• Existing regions still depend on the legacy /voice_agents API and object-shaped audio.output.voice payload.
• West US 2 TiP now supports the unified /agents API and the newer flat audio.output.voice shape from the latest REST spec.
• This PR creates a safe API migration layer before rebasing the advanced prompt voice settings work on top of the unified API.

Validation

• go test ./... -count=1
• go build ./...
• cspell lint 'extensions/**/*.go' 'extensions/**/*.md' --config ./.vscode/cspell.yaml --no-progress
• Live West US 2 TiP smoke with AZURE_VOICE_AGENT_API=unified-flat: provision, deploy, repeat deploy via unified update path, GET assertion for flat output shape and /agents/{name}/endpoint/protocols/voice endpoint.
• Adapted full prompt voice test guide flow in West US 2 with AZURE_VOICE_AGENT_API=unified-flat: managed agent init/provision/deploy, BYOM realtime and BYOM cascaded model deployments, BYOM service init/deploy, unified /agents/{name} control-plane GET assertions for managed/self_deployed model types and flat output shape, same-agent redeploy through unified update path, and voice endpoint route smoke (400 expected for non-WebSocket GET).
• Legacy-to-unified migration smoke in West US 2: first deploy used default legacy mode and /voice_agents, then AZURE_VOICE_AGENT_API=unified-flat redeployed the same agent through /agents/{name} update and returned flat output shape.
• Guide test resources were cleaned up; resource groups rg-voice-guide-managed-dev-059e8e96 and rg-voice-migration-smoke-dev-2cd56f1e were deleted.

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