[layers v2] azd down, with a single layer

With layers, we can now have something like this:

layers:
- name: layer-with-only-infra
infra:
- name: infra-1
- name: infra-2
- name: layer-with-services-and-infra
infra:
- name: infra-3
- name: infra-4
services:
servicea:
serviceb:
- name: services-only-layer
services:
servicea:
serviceb:

There are some gaps here that become more apparent with layers, and we need to determine how azd should handle them.

Today, azd cannot target and delete an individual service deployment. You can destroy the IaC-managed infrastructure hosting the service, which also removes the service, but you cannot remove only the deployed service while preserving infrastructure that might be shared with other services or contain other important resources.

This becomes more significant with Microsoft Foundry, where one Foundry project can host multiple independent agents. Destroying the Foundry project or its surrounding infrastructure to remove one agent would also remove the other agents and project-scoped assets.

The Azure AI Agents extension addresses this with its own command, azd ai agent delete. We should consider making service deletion a generic azd operation that can target a service and delegate its teardown to the corresponding built-in or extension-provided service target.

This could also be useful for existing service types such as Azure Functions. We need to decide whether deleting a service means removing only its deployed artifact, deleting its hosting resource, or supporting both operations explicitly.

CC: @trangevi, to double-check that I got the Foundry details correct.