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test(runtime): add test coverage for ValidateObjects methods

Add comprehensive test coverage for TrainingRuntime.ValidateObjects()
and ClusterTrainingRuntime.ValidateObjects() methods which previously
had zero test coverage.

Tests cover existing behavior only:
- Runtime exists and is valid (success path)
- Runtime does not exist (returns field error)
- Runtime in different namespace (returns field error for TrainingRuntime)
- Deprecated runtime (returns warning for ClusterTrainingRuntime)

All tests verify the current implementation without proposing new
validation rules.

Signed-off-by: Prateek Rai <prateekrai903@gmail.com>