feat: add K3s node detection to kube endpoint

[atheurer]

Summary

• Add detection for K3s control-plane nodes using the node-role.kubernetes.io/control-plane label
• K3s single-node clusters are treated the same as microk8s (node counted as both master and worker)
• Enables kube endpoint to work with K3s distributions

Problem

When using K3s, the kube endpoint was failing with "No masters found" and "No workers found" errors because it only had node detection logic for standard k8s and microk8s distributions.

Solution

Added K3s node detection following the same pattern as microk8s (single-node treated as both master and worker), just using K3s's node-role.kubernetes.io/control-plane label instead of microk8s's node.kubernetes.io/microk8s-controlplane label.

Test plan

• Verify existing microk8s support still works (CI should validate this)
• Test with K3s clusters to confirm node detection works
• Confirm no regression for standard k8s deployments

🤖 Generated with Claude Code

[atheurer]

@k-rister if this works, I assume we will have to backport to the stable releases, since the upcoming CI changes would be using runners with k3s, and those stable releases use the same CI. There may be an earlier rickshaw PR we have to backport as well.

[k-rister]

Yeah, we will probably have to backport this, but I don't think it will be to all 4 releases. I'll have to look at which one(s).

[k-rister]

Yeah, we will probably have to backport this, but I don't think it will be to all 4 releases. I'll have to look at which one(s).

I think it's just going to be 2025.4 and 2026.1