cache: add prune until parent regression test

[maxpetrusenkoagent]

Adds regression coverage for pruning old retained parent/child cache refs with a keep-duration filter.

This covers the behavior reported in #5436 where pruning an old child should not leave its old parent behind.

Focused verification:

• go test ./cache -run 'TestPruneUntilRemovesOldParentAfterChild|TestManager' -count=1 -v\n\nNote: go test ./cache -count=1 was also attempted on macOS, but existing platform-sensitive tests failed (TestSnapshotExtract clonefile permission denied, TestDiffOp diffApply unsupported on darwin, TestMergeOp size mismatch).\n\nFixes docker buildx prune --filter="until=xyz" marks unrelated cache layers as "last used" and does not delete parents #5436