[fix] validate pni crd is immutable

[aranta-rokade]

Reason for Change:
Make PNI CRD immutable using Kubernetes validations

Issue Fixed:
Making PNI CRD immutable, allows PNIs to retry reconcile after failure, for the cases when IP allocations become available.

• uses conventional commit messages
• includes documentation
• adds unit tests
• relevant PR labels added

Tested

k --kubeconfig=cluster.yaml apply -f ../pni.yaml
The PodNetworkInstance "pni-10" is invalid: <nil>: Invalid value: "object": Spec is immutable.

[Copilot]

Pull request overview

This PR adds immutability validation to the PodNetworkInstance CRD's spec field using Kubernetes CEL validation rules. The goal is to prevent modifications to the PNI spec after creation, which allows PNIs to retry reconciliation after failures when IP allocations become available.

• Adds kubebuilder XValidation annotation to enforce self.spec == oldSelf.spec
• Updates generated CRD manifest with x-kubernetes-validations
• Adds documentation comments explaining the validation rule and its requirements

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
crd/multitenancy/api/v1alpha1/podnetworkinstance.go	Adds kubebuilder XValidation annotation to PodNetworkInstance type to enforce spec immutability
crd/multitenancy/manifests/multitenancy.acn.azure.com_podnetworkinstances.yaml	Updates generated CRD manifest with x-kubernetes-validations rule to enforce spec immutability

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