IPv6 control-plane endpoint: join URL is not bracketed #1565

What happened?

Workers never join when the control-plane Service has an IPv6 LoadBalancer address:

unable to create certificates signing request client: host must be a URL or a host:port pair: "https://xxxx:xxxx:11:1103:t::3:443"

internal/controller/bootstrap/worker_bootstrap_controller.go:532 (also :534, and
controlplane_bootstrap_controller.go:698 and :710) build the URL with
fmt.Sprintf("https://%s:%d", host, port). An IPv6 literal needs brackets —
net.JoinHostPort adds them, which is what k0s does in pkg/apis/k0s/v1beta1/api.go.

The endpoint itself is fine: curl -sk 'https://[https://xxxx:xxxx:11:1103:t::3]/version' → 401.

Involved Areas

Cluster API integration

What did you expect to happen?

The worker joins. The join URL has to be https://[https://xxxx:xxxx:11:1103:t::3]:443 — an IPv6 literal in a URL needs brackets.

Steps to reproduce

1. Dual-stack management cluster with an IPv6 LoadBalancer pool (MetalLB here).
2. K0smotronControlPlane with service.type: LoadBalancer, apiPort 443, and a Service that ends up single-stack IPv6, so the VIP is an IPv6 address.
3. Create a worker Machine.
   The control plane comes up and answers on https://[]:443, the worker aborts while bootstrapping the kubelet.

k0smotron version

2.0.4

k0s version

1.36.2+k0s.0

Anything else we need to know?

No response