🔧 pytest fixtures that actually scale: patterns from 2 years of Python CI pipelines

I spent two years watching the same test suite failures in CI.

Not logic failures — the test logic was fine. Infrastructure failures. The fixture that created a database table but didn't clean it up. The S3 mock that shared state across test files and produced a passing suite locally and a flaky one in CI. The fixture that took 4 seconds to set...