[Repo Assist] [chore]: add GKE and AKS cloud providers to extend KSail beyond AWS EKS

[github-actions]

User Story

As a KSail user running services on Google Cloud (GKE) or Microsoft Azure (AKS),
I want to use ksail cluster create --distribution Vanilla --provider GKE (or AKS),
So that I can manage production-grade cloud Kubernetes clusters with KSail's embedded toolchain and GitOps workflows — the same way I do locally.

Context

After AWS EKS (#4328) ships, GKE and AKS are the natural next cloud targets. Together they cover the three most common enterprise Kubernetes platforms. KSail already has a provider abstraction (pkg/svc/provider/) that isolates distribution logic from cloud lifecycle management, so the pattern established by the Hetzner and AWS providers can be reused.

Distribution	Docker	Hetzner	Omni	AWS	GKE	AKS
Talos	✅	✅	✅	—	—	—
Vanilla / K3s / VCluster	✅	—	—	—	—	—
EKS (managed)	—	—	—	🚧	—	—
GKE (managed)	—	—	—	—	❌	—
AKS (managed)	—	—	—	—	—	❌

Acceptance Criteria

• ksail cluster create --provider GKE provisions a GKE cluster using the Google Cloud SDK
• ksail cluster create --provider AKS provisions an AKS cluster using the Azure SDK
• Full lifecycle (create, update, delete, start, stop, info) works for both providers
• Distribution–provider validation updated in pkg/apis/cluster/v1alpha1/errors.go
• CI smoke tests for each new provider path
• Documentation updated with GKE and AKS setup guides

Notes

• Priority: Later — depends on AWS EKS ([Repo Assist] [chore]: complete AWS EKS provider #4328) completing first
• Complexity: High — requires Google Cloud and Azure SDK integration, cluster lifecycle management, kubeconfig refresh
• Track GKE and AKS separately if implementation timelines diverge

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