Advisory check that the subscription's service principal can provision session host VMs:
VM, NIC, and disk creation, VM extensions, run commands, subnet join, gallery image read,
and host pool registration-token generation. Covers three flows: initial session hosts at
host pool creation (host count greater than zero), POST session-host-deployments, and
autoscale scale-out.

Each required permission is evaluated at exactly the scope the provisioning flow exercises
it against (inherited grants from resource group, subscription, or management group count).

Always returns 200 for a valid request and subscription and never blocks creation:

• granted: all required actions confirmed at their target scopes.
• missingPermissions: at least one action could not be confirmed; display the returned message as a warning.
• unknown: the check itself could not run (transient error); do not show a warning.