Add comprehensive unit tests for pkg/itemblock which previously had
zero test coverage. The new tests cover AddUnstructured and FindItem
methods with 14 test cases across 3 test functions, including preferred
GVR ordering, nil item handling, namespace/name filtering, unparseable
apiVersion handling, and cluster-scoped resource matching.
Fixes#10418
Signed-off-by: opbot_xd <awasthikrishna23052005@gmail.com>
