Census: every Darling live test class skips when DARLING_TEST_PG is unset #5759

Claude posting for Erik Darling.

Nothing checks that a Darling live test class skips when DARLING_TEST_PG is unset. #5726 added two live classes without the skip (ExternalAlertDrainLiveTests, ExternalAlertDrainLifecycleLiveTests). Their PR passed, but dev's whole-tree guards job, which runs Darling.Tests with no store by design, then failed 34 tests with ArgumentNullException (Parameter 'Host') (#5755, fixed in #5756).

The existing checks cover collection membership (LivePostgresCollectionHygieneTests), cluster objects, clocks, plan isolation, unpooled drops and scratch-database disposal. None of them covers the skip.

Done when a census test fails for a class that creates a scratch database (ScratchPostgres.CreateAsync or the shared fixtures) on a path that does not first skip on an unset DARLING_TEST_PG. The census:

• is planted red on a copy of the pre-Dev CI: UiSmokeWpf lock file pins xunit.v3 4.0.2, and two live test classes skip without DARLING_TEST_PG (#5755) #5756 drain class;
• passes on dev.