feat: implement CAPI failure domain contract for zones

[emanuelebosetti]

Issue: #717
Description of changes:

Map existing CAPMOX zones to CAPI failure domains so that KubeadmControlPlane automatically distributes control plane nodes across Proxmox zones.

API changes (additive, backwards-compatible):

• Add Nodes []string to ZoneConfigSpec for per-zone node lists
• Add ControlPlane *bool to ZoneConfigSpec (defaults to true)
• Add FailureDomains to ProxmoxClusterStatus (CAPI v1beta2 contract)
• Add spec.failureDomain to ProxmoxMachineSpec (InfraMachine contract)
• Add GetZoneNodes() helper on ProxmoxCluster
• Add FailureDomainNotReady condition reason

Controller changes:

• Add reconcileFailureDomains in cluster controller (sorted by name to prevent reconcile loops)
• Read Machine.Spec.FailureDomain in machine controller with retryable FailureDomainNotReady condition
• Use effectiveZone/effectiveAllowedNodes on MachineScope to avoid spec mutation from the controller
• Pass zone override to GetInClusterPools for IPAM pool selection
• Set Zone on NodeLocation in createVM
• Update scheduler to use GetEffectiveAllowedNodes()

Conversion:

• Restore Status.FailureDomains and Spec.FailureDomain in v1alpha1 conversion webhook
• Add manual conversion stub for ProxmoxMachineSpec.FailureDomain

Testing performed:

• Unit tests for API types, scheduler, VM service helpers, and machine scope
• Validated on a real Proxmox cluster (10 nodes, 2 zones) with 3 CP + 2 worker nodes
• KCP correctly distributes CP replicas round-robin across failure domains
• Workers pinned to target zone, IPAM pools selected per-zone
• make lint test verify passed
• Full backwards compatibility verified — clusters without zoneConfig work as before

[wikkyk]

Thanks!
If you could please provide attribution if you used AI/coding agent(s) to develop your PR.

[emanuelebosetti]

I've amended the commit to include the Co-Authored-By trailer for the AI coding agent used during development.
Let me know if there are any additional attribution policies or guidelines I should follow for AI-assisted contributions to this project.

[wikkyk]

Thanks. No, we don't have any other policies. The field moves fast and we just didn't get to writing things down yet.

[emanuelebosetti]

Thanks. No, we don't have any other policies. The field moves fast and we just didn't get to writing things down yet.

We're going through the same process internally — introducing AI across dev teams and figuring out contribution policies.

Three references we found particularly useful:

• CNPG AI Policy
• Ghostty AI Policy
• Linux Foundation Generative AI Policy

Could be useful input for #716 as well.

[emanuelebosetti]

Hi @wikkyk , do you think it would be possible to include these failure domain changes in the upcoming v0.9.0 release? Happy to address any remaining feedback if needed. Thanks!

[wikkyk]

Sure. v0.9.0 is still a fair bit far off, though. This only adds a field so it could reasonably be released in the 0.8 series.

However, as the linter points out, the type of the FailureDomain field has the wrong type. It's optional, so it should be a pointer. You'll need to re-check your pointer handling.

[emanuelebosetti]

@wikkyk pushed the pointer fix as a separate commit. Could you re-trigger the workflows? Thanks!