Remove obsolete tests and fix 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.

Merged #7238: alembic head pin (142 edits/14d, a CI red source) replaced by a chain invariant; 2 obsolete tests deleted; one measured vitest speedup. All recent CI reds were already fixed on develop; test.yml green. Deferred: CI pycache (~30-40s). Memory: none applied.