[Multi_K8s-Plugin] Add config validation for KubernetesApplicationSpec

[mohammedfirdouss]

What this PR does / Why we need it:

Implements KubernetesApplicationSpec.Validate() in the multicluster Kubernetes plugin. The method was previously a stub that always returned nil, meaning invalid configs passed silently at load time and only failed mid-deployment with cryptic errors.

Two validations are added:

1. helmChart and kustomizeOptions are mutually exclusive — only one templating method can be used. Specifying both previously resulted in undefined/silent behaviour.

2. Each multiTargets[].target.name must not be empty — an empty target name causes the deploy target lookup to fail at runtime. This catches it before any stage runs.

Which issue(s) this PR fixes:

Part of #6446

How was this tested:

Unit tests added to config/application_test.go covering:

• Empty spec (valid)
• Valid multiTargets
• multiTargets with an empty target name (error)
• helmChart + kustomizeOptions together (error)
• Each individually (valid)

Note for single-cluster kubernetes plugin: pkg/app/pipedv1/plugin/kubernetes/config/application.go has the same Validate() stub (introduced in #5356). The same two validations apply there, plus the single-cluster plugin does not have multiTargets so only the helm/kustomize mutual exclusion check is relevant.

cc @Warashi you introduced the original stub in the single-cluster plugin, flagging in case you want to track this.

[Warashi]

LGTM

[github-actions]

Thank you for contributing to PipeCD! The changes in this pull request will be part of the upcoming release!