fix(kyaml): correctly handle patches with a partial compound merge key

[SetagGnaw]

What this fixes

When a strategic merge patch targets an element of an associative list by only
part of a compound merge key, kustomize loses or corrupts data. With the
[port, protocol] merge key for Service ports ([containerPort, protocol] for
container ports), a patch that omits protocol:

• loses an added field when it matches an existing port and adds a field;
• drops the partial entries entirely when the patch lists several ports and
  some omit protocol while sharing a port number with a sibling that supplies
  it (patchesStrategicMerge is not working for service of single port but multiple protocols. #4752); and
• fails to update the matching element and leave its sibling intact when the
  base holds two elements sharing the port (e.g. 8080/TCP and 8080/UDP).

Root cause

kustomize resolves an associative list's merge key to the compound
x-kubernetes-list-map-keys (e.g. [port, protocol]). A patch that specifies
only part of that key was mishandled in setAssociativeSequenceElements
(kyaml/yaml/walk/associative_sequence.go): the partial key either could not be
re-matched, so the element's data was dropped, or it matched every element
sharing the specified component, clobbering a sibling that differs only by
another component.

The fix

Match each element on the key it actually specifies:

• when the element populates the full compound key, use it;
• otherwise fall back to the single strategic-merge-patch merge key the schema
  declares for the list (x-kubernetes-patch-merge-key, e.g. port), which is
  the key the SMP path is meant to use.

A merged element is then re-keyed on the full key it now populates
(elementKeyValues) so it replaces its exact counterpart instead of every
element sharing the partial key, and the full compound key is passed to
appendListNode.

Net behavior: a partial-key patch merges into a single matching element (or
appends as a distinct element when none match), and same-port siblings are
preserved rather than clobbered or duplicated.

Testing

Three cases added to TestMerge in kyaml/yaml/merge2/map_test.go, all failing
before the fix and passing after:

• port patch adds a field but omits protocol (partial compound key)
• patch with multiple ports, some omitting protocol (#4752)
• partial-key patch when base has two elements sharing the port (ambiguous)

go test ./yaml/merge2/... and the full kyaml suite pass. Two-commit
bug-fix convention: the first commit adds the failing tests, the second applies
the fix.

Related issues

Fixes #4752

• Merge ContainerPort ports field using containerPort,protocol tuple #3111 introduced the containerPort,protocol compound (list) merge key
  (implemented in use merge key list instead of a single merge key #3159, shipped v3.8.7); this PR fixes data-loss and clobbering
  bugs in that merge path.

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[kubernetes-prow]

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[kubernetes-prow]

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