kyaml: fix $patch directives ignored with ListPrepend merge

[rm3l]

Summary

Fixes #6146

• $patch: replace and $patch: delete directives on individual associative list elements were silently ignored when ListIncreaseDirection was set to MergeOptionsListPrepend
• Root cause: in setAssociativeSequenceElements, the ListPrepend path called appendListNode(itemsToBeAdded, dest, ...) with swapped arguments, causing ElementSetter to overwrite already-merged items with unmerged originals from dest
• Fix: introduce appendMissingListNode that only appends elements not already present in the destination, preserving merged results

NOTE:
I've created a branch in the repro repo using the patched kyaml from this PR, and it seems to be fixing the issue: https://github.com/rm3l/kyaml-listprepend-bug/tree/fix/use-patched-kyaml

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