Conditional Access blocks authentication due to unknown device platform

Entra ID Conditional Access enables tenants to block authentication based on originating device platform as determined by User-Agent strings ([feature documentation](https://learn.microsoft.com/entra/identity/conditional-access/concept-conditional-access-conditions#device-platforms)). Azure CLI evidently doesn't set User-Agent for its token requests--they have the default from `requests` e.g. `python-requests/2.31.0`. This doesn't identify any platform, so Conditional Access considers such requests to originate from unknown platforms. The Azure CLI therefore can't authenticate in a tenant having a policy blocking unknown platforms even when used from a known and allowed platform. (From Azure/azure-sdk-for-go/issues/22155)