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fix(controller): validate TrainJob updates against the runtime snapshot

ValidateObjects always fetched the live (Cluster)TrainingRuntime, so editing
or deleting a runtime could retroactively break validation for an existing
TrainJob. Resuming a suspended TrainJob whose runtime had changed, or whose
runtime had been deleted, was rejected by the webhook even though
reconciliation would have succeeded from the snapshot.

Resolve the runtime from the snapshot on update, falling back to the live
object when no snapshot exists, mirroring NewObjects. The existence check,
the deprecation-label warning and RunCustomValidationPlugins all read from
the resolved runtime.

Fixes #3923

Signed-off-by: Abhishek <abhikokadwar2@gmail.com>