cumulus: bump PARENT_SEARCH_DEPTH and add test for 12-core elastic scaling #16810

Job	Run time
check-dependency-rules	10s
check-fail-ci	16s
check-markdown	13s
check-readme	3m 6s
check-workspace	16s
preflight / preflight	45s
preflight / ci-versions	55s
fmt	1m 6s
check-rust-feature-propagation	1m 55s
check-toml-format	58s
test-rust-features	1m 56s
check-umbrella	2m 10s
All quick checks passed	1s
	13m 47s

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