Function fetch_account_email 

pub async fn fetch_account_email(
    provider: CloudProvider,
    tokens: &OAuthTokens,
) -> Result<String, OAuthError>

Fetch the email of the account tokens authenticated, for the given OAuth provider. The joining device calls this right after authenticating so the approver can share the OAuth folder to its provider-account email.

Only the OAuth providers are valid here; a non-OAuth provider (S3, CloudKit) is a programming error and surfaces as an error rather than a silent default.