Review and fix test suites and CI failures

Review all test suites, delete stale, superseeded, those that were one off to check feature flags or absence of something. Kill the most long running or flaky with unit tests that execute in milliseconds. Regroup between quick and full, Fix broken tests in CI - GitHub Actions - look at a few recent test runs and fix broken/flaky tests. Babysit CI/CD until the tests (test.yaml) pass.

Fixed the one live CI flake (SSE keepalive test now checks its deadline without a wall-clock bound; a mutation still fails it). Every other recent red was already fixed on develop. Sweeps found nothing to delete. #6800 merged and Jaah Deploy is green. No promotion PR is open for test.yml to run on.