fix: inject Katib labels early to support PodDefaults for trial pods

[Priyanshu-u07]

When a trial pod is created, Katib labels like katib.kubeflow.org/trial are added too late . They get added by a webhook that runs after the PodDefaults webhook has already finished checking. Because of this timing issue, PodDefaults that depend on Katib labels are never applied to trial pods.

This PR fixes it by adding the labels directly into the pod template before the pod is created, so PodDefaults can see them in time.

Changes:

• Added InjectLabelsToPodTemplate utility in pkg/controller.v1beta1/util/unstructured.go that injects labels into Pod templates across workload types (Job, CronJob, Training Operator replicaSpecs)
• Updated generator.go to inject experiment labels into the trial template early
• Updated trial_controller.go to inject trial name labels into the workload before creation
• Added unit tests covering all supported workload types, including edge case where no initial labels exist

Fixes #2548

[github-actions]

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[google-oss-prow]

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[Priyanshu-u07]

@andreyvelich Could you please take a look when you get a chance?
Thanks!