Clean up and fix broken test suites

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.

CI test.yml green (CI failures were already fixed upstream). PR #6819 merged and deployed: retired applied one-shot migration tests, removed an unsupported 360px e2e arm, measured all suites. CI Playwright 2→4 workers deferred: unmeasurable from a runner. Memory: none applied.