fix(runtimes): propagate Trainer.NumNodes into TemplateSpec (Parallelism/Completions)

[NarayanaSabari]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This PR fixes the RuntimeInfo helpers to correctly synchronize PodSets.Count values (set by numNodes) to the TemplateSpec.ObjApply (JobSet specification).

Previously, when external consumers like Kueue called RuntimeInfo(), the info.TemplateSpec.PodSets[*].Count was correctly updated by ML policy plugins (Torch, PlainML, MPI), but the underlying JobSet spec (info.TemplateSpec.ObjApply) still had the original Parallelism and Completions values. This required consumers to implement post-processing workarounds.

Changes:

1. Added SyncPodSetsToTemplateSpec() method in runtime.go that syncs PodSets.Count to JobSet's Parallelism/Completions
2. Called this method in RuntimeInfo() after RunEnforceMLPolicyPlugins() runs
3. Added unit tests for the new sync function

Which issue(s) this PR fixes:

Fixes https://github.com/kubeflow/trainer/issues/3042

Related Kueue issue: https://github.com/kubernetes-sigs/kueue/issues/8287

Special notes for your reviewer:

• The existing comment in runtime.go line 45 already mentioned: "ObjApply podSpecs and this PodSets should be kept in sync by info.SyncPodSetsToTemplateSpec()" - this PR implements that function.
• Once this is merged, the workaround in Kueue's trainjob_controller.go can be removed.
• All existing tests pass.

[google-oss-prow[bot]]

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[andreyvelich]

/ok-to-test