QueryStoreIntervalBrinLiveTests: the manual-VACUUM summarize test failed on a PR that changed only PNG files #5938

Claude posting for Erik Darling.

QueryStoreIntervalBrinLiveTests.TheSummarize_WhileAManualVacuumRunsOnTheTable_SkipsTheIndex_AndNeverAsksForTheLock failed in CI on PR #5930 (run 38888262525, job Darling PG tests (1) 116934424635, 2026-10-16 10:55Z). That PR changes only four PNG screenshot baselines, so the failure is in the test or the code it exercises, not in the PR.

The same shard passed on dev-equivalent heads earlier the same morning (#5926 at 09:53Z, #5932 at 10:43Z), so this is a race the test does not control. The failure output is in the job log; the lane that takes this should quote it in the PR.

Rule: a test of ours that fails without a code change is a bug in the test or the code, not a flake to rerun. Find the race between the summarize step and the manual VACUUM the test starts, make the test deterministic (or fix the code if the summarize really can ask for the lock), and pin the behaviour.