Improve how AI_PROJECT_DEPLOYMENTS is used vs. agent.yaml env vars

An agent manifest defines a model resource like: https://github.com/microsoft-foundry/foundry-samples/blob/main/samples/python/hosted-agents/agent-framework/agents-in-workflow/agent.yaml#L23-L24

The agent code likely uses that by reading `AZURE_OPENAI_CHAT_DEPLOYMENT_NAME` but we never set an env var with that name in the azd environment variables. Instead, deployments are put in json in a `AI_PROJECT_DEPLOYMENTS` variable.