feat(podgrouper): support owner-chain fallback for plugin selection

[singh1203]

Description

feat(podgrouper): Implement owner-reference chain fallback for plugin selection

Enhance PodGrouper.GetPGMetadata() to iterate through the owner chain (allOwners slice)
from top-most owner down, selecting the highest GVK with a non-default plugin via
PluginsHub.GetPodGrouperPlugin(). Eliminates scheduling regressions when new owner types
gain RBAC visibility but lack dedicated plugins (e.g., JobSet → fallback to batch/v1.Job).

Related Issues

Fixes #856

Checklist

• Self-reviewed
• Added/updated tests
• Updated documentation

Breaking Changes

None

Additional Notes

Before: JobSet → no plugin → defaultgrouper (per-pod grouping)
After: JobSet → no plugin → Job → job.NewK8sJobGrouper (Job-level grouping)

Implementation via selectPluginForOwners() helper using DefaultPluginsHub type assertion.
No PluginsHub interface or plugin signature changes required.

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/NVIDIA/KAI-scheduler/pkg/podgrouper/podgrouper	73.17% (+1.74%)	👍

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/NVIDIA/KAI-scheduler/pkg/podgrouper/podgrouper/podgrouper.go	73.17% (+1.74%)	82 (+12)	60 (+10)	22 (+2)	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/NVIDIA/KAI-scheduler/pkg/podgrouper/podgrouper/podgrouper_test.go