kyaml merge2: `$patch` directives on list elements ignored with `ListPrepend`

/kind bug

Bug

When using merge2.MergeStrings with MergeOptions{ListIncreaseDirection: MergeOptionsListPrepend}, strategic merge patch directives ($patch: replace, $patch: delete) on individual associative list elements are silently ignored.

With default MergeOptions{} (Append), $patch: replace correctly replaces the matched list element. With ListPrepend, the original element remains unchanged and the replacement is dropped.

Root cause

In kyaml/yaml/walk/associative_sequence.go, setAssociativeSequenceElements() assembles the final list differently depending on the merge direction:

• Append (correct): appendListNode(dest, itemsToBeAdded, ...); merged results from itemsToBeAdded overwrite originals in dest.
• ListPrepend (buggy): appendListNode(itemsToBeAdded, dest, ...); the arguments are swapped for ordering, but this causes ElementSetter to overwrite already-merged items in itemsToBeAdded with the unmerged originals from dest, undoing $patch directive effects.

Reproduction

Minimal reproduction project: https://github.com/rm3l/kyaml-listprepend-bug

git clone https://github.com/rm3l/kyaml-listprepend-bug
cd kyaml-listprepend-bug
go test -v ./...

• TestPatchReplaceWithDefaultMerge: PASS
• TestPatchReplaceWithListPrepend: FAIL (the $patch: replace is ignored)

Versions affected

Tested and confirmed on:

• sigs.k8s.io/kustomize/kyaml v0.18.1
• sigs.k8s.io/kustomize/kyaml v0.19.0
• sigs.k8s.io/kustomize/kyaml v0.21.0
• sigs.k8s.io/kustomize/kyaml v0.21.1
• Latest master commit (313aaced, 2025-05-02)

Impact

This bug prevents combining ListPrepend (for controlling list item ordering) with $patch: replace or $patch: delete (for replacing/removing individual items by merge key). The two features are independent and should compose correctly IMHO.