Warning on missing placeholders still mentions agent.yaml

_Migrated from [huimiu/azd-foundry-extension-bugbash#5](https://github.com/huimiu/azd-foundry-extension-bugbash/issues/5), originally reported by @therealjohn._

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`Warning: agent.yaml has 1 unresolved placeholder(s): TOOLBOX_ENDPOINT. Edit agent.yaml and replace each `{{NAME}}` with the actual value before deploying.`

agent.yaml is no more. 
