Real Proxmox's POST /nodes/{node}/storage/{storage}/upload only accepts
content in {iso, vztmpl, import} — content='snippets' is rejected
outright with a 400 ("does not have a value in the enumeration").
Snippets can only be written directly to the storage's filesystem path.
Found live, right after the previous multipart-upload fix: the VM
shell, disk import, and node-scoped storage selection all succeeded,
then create_vm_from_cloud_init failed with a 400 at the snippet write
step. Resolves the storage's path via the cluster storage config and
writes the file over SSH (base64-piped, to survive arbitrary YAML
content safely).