Azure App Configuration 403 forbidden accessing with managed identity DefaultAzureCredential

## The problem

A function app with a user-assigned managed identity that has IAM Reader on the App Configuration store and Get/List on Key Vault secrets gets 403 forbidden when accessing App Configuration through DefaultAzureCredential, even though the identity looks correctly permissioned.

## The fix

IAM Reader is not enough: App Configuration data access requires a data-plane role. Assign the identity either the Azure App Configuration Data Reader or Azure App Configuration Data Owner role on the store. (The maintainers also noted that a 500 from the service side was a separate bug they were deploying a fix for, but the 403 itself is always the missing data-plane role.)