Test suite cleanup and CI fixes

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.

Found why test-stack was red: billing reset leak (2 API reds) and 3 smoke regressions from Task 8687. Same fixes landed on develop (8720/8725); verified locally (API/vitest/smoke all green). PR #6534 queued: 2 rejected perf levers with measurements. No test.yml run since 13:19Z. Memory: proposed 1 gotcha