Fixes #39142 - skip compute orchestration for unmanaged hosts

[jakduch]

When a host is registered via Global Registration (managed: false), the compute orchestration in queue_compute still runs, attempting to create/update a VM on the compute resource. This causes errors for hosts that are not meant to be managed by Foreman's compute layer.

This patch adds a managed? guard to queue_compute so that compute orchestration is skipped for unmanaged hosts.

The queue_compute_destroy method intentionally does NOT get this guard — VM cleanup should work for all hosts regardless of managed status, as confirmed by existing tests in Host::ManagedTest::deletion.

Testing:

• Register a host via Global Registration with a compute resource assigned
• • Verify that compute orchestration does not run for the unmanaged host
• • Verify that managed hosts still get compute orchestration as expected
• • Verify that VM deletion still works for all hosts