azure.ai.agents - `azd down` behavior in existing project scenarios

There are some opportunities to improve the `azd down` experience for existing project scenarios (using `azd ai agent init` with `--project-id` flag). Here are a set of repro steps highlighting one issue:

### Part 1 - In `/workspaces/calculator-new`:

1. `azd init -t Azure-Samples/azd-ai-starter-basic` with environment name **`calculator-new`**
2. `azd ai agent init -m https://github.com/azure-ai-foundry/foundry-samples/blob/main/samples/microsoft/python/getting-started-agents/hosted-agents/calculator-agent/agent.yaml`
3. `azd up`

This creates a resource group `rg-calculator-new` with following resources:
- AI Foundry account
- AI Foundry project - `/subscriptions/sub-a/resourceGroups/rg-calculator-new/providers/Microsoft.CognitiveServices/accounts/ai-account-qunquemu56q76/projects/ai-project-calculator-new`
- Application Insights
- Container registry - `crqunquemu56q76.azurecr.io`
- Log Analytics workspace

Both the resource group and its child resources have the Azure tag applied: `azd-env-name: calculator-new`.

### Part 2 - In a different directory `/workspaces/calculator-existing`:

1. `azd init -t Azure-Samples/azd-ai-starter-basic` with environment name **`calculator-existing`**
2. `azd ai agent init -m https://github.com/azure-ai-foundry/foundry-samples/blob/main/samples/microsoft/python/getting-started-agents/hosted-agents/calculator-agent/agent.yaml --project-id /subscriptions/sub-a/resourceGroups/rg-calculator-new/providers/Microsoft.CognitiveServices/accounts/ai-account-qunquemu56q76/projects/ai-project-calculator-new` and choose existing model deployment.
- This sets the following environment variables in `.azure/calculator-existing/.env`:
```
AZURE_AI_ACCOUNT_NAME="ai-account-qunquemu56q76"
AZURE_AI_PROJECT_ENDPOINT="https://ai-account-qunquemu56q76.services.ai.azure.com/api/projects/ai-project-calculator-new"
AZURE_AI_PROJECT_ID="/subscriptions/sub-a/resourceGroups/rg-calculator-new/providers/Microsoft.CognitiveServices/accounts/ai-account-qunquemu56q76/projects/ai-project-calculator-new"
AZURE_AI_PROJECT_NAME="ai-project-calculator-new"
AZURE_CONTAINER_REGISTRY_ENDPOINT="crqunquemu56q76.azurecr.io"
AZURE_ENV_NAME="calculator-existing"
AZURE_LOCATION="southcentralus"
AZURE_OPENAI_ENDPOINT="https://ai-account-qunquemu56q76.openai.azure.com/"
AZURE_RESOURCE_GROUP="rg-calculator-new"
AZURE_SUBSCRIPTION_ID="sub-a"
```
4. `azd up`
- This doesn't provision any new resources. There is no new `rg-calculator-existing` resource group created either, as `AZURE_RESOURCE_GROUP` was set to `rg-calculator-new` in the previous step.
- During provision, the tag `azd-env-name` on the `rg-calculator-new` resource group and its resources gets changed to `calculator-existing`.
6. `azd down`
- azd finds the resource group `rg-calculator-new` has the tag `azd-env-name` with value `calculator-existing` and prompts for deletion: