[Feature] Support `azd deploy` to a named App Service deployment slot (host: appservice)

[qiaozha]

Summary

azd only models the production slot of an App Service. There is no way to bind an azd service to a named deployment slot, so azd deploy <service> cannot target a slot.

What happens

We run a Python FastAPI "agent server" (server.py) in the agent deployment slot of our backend App Service. Running:

azd deploy agent-server

fails at the core deploy step with:

unable to find a resource tagged with azd-service-name: agent-server

because the only resource carrying that tag is the production site, not the slot.

Current workaround

A per-service predeploy hook (agent-server-predeploy.ts) does the deploy itself:

• az acr build -> immutable tag dev-N.0.0
• az webapp config container set --slot agent
• writes the immutable tag back to .env so the next azd provision re-pins the slot (the Bicep default :dev tag drifts)

The azd deploy agent-server core step then becomes a tolerated no-op — all the real work is in the hook.

Ask

First-class named-slot support for the appservice host, e.g. a way in azure.yaml to declare that a service deploys to a specific slot, so azd deploy targets the slot natively. This would let us delete agent-server-predeploy.ts.

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Context: encountered while deploying a multi-component AI/chat service (App Service + Function App + Logic App + Foundry hosted agent) end-to-end with azd provision/deploy in Azure/azure-sdk-tools tools/sdk-ai-bots/deployment. We currently work around this with a TypeScript hook.