Add config validation for KubernetesApplicationSpec in pipedv1

[ayushsarode]

What would you like to be added:

Implement proper validation for KubernetesApplicationSpec in the pipedv1 Kubernetes plugin (pkg/app/pipedv1/plugin/kubernetes/config), replacing the current // TODO: Validate KubernetesApplicationSpec fields. stub.

Validation should cover at least:

1. Mutually exclusive input options

   • Reject configs that set both input.helmChart and input.kustomizeOptions.

2. Traffic routing method

   • When trafficRouting is set, allow only supported methods ("", podselector, istio) and fail clearly on unknown values.

3. Variant labels

   • Require non-empty variantLabel.key, primaryValue, canaryValue, and baselineValue.
   • Require primaryValue, canaryValue, and baselineValue to be unique.

Add unit tests for valid configs, each failure case, and that JSON unmarshaling applies defaults so a minimal {} spec remains valid after defaults are set.

Why is this needed:

Invalid Kubernetes application configs are currently accepted and only fail later during plan/sync (or produce confusing progressive-delivery behavior). Early validation:

• Surfaces configuration mistakes immediately with actionable error messages.
• Prevents impossible combinations (e.g. Helm chart + Kustomize options).
• Protects progressive delivery by ensuring variant labels and traffic-routing method are usable before a deployment starts.
• Aligns the single-cluster Kubernetes plugin with existing validation patterns used by other plugins (e.g. multicluster Kubernetes, analysis, terraform).