conftest: add readiness check before test session

Poll each service base URL until it responds with a non-502/503 status
before the test session begins. Retries on:

  - ConnectionError: process not yet bound to its port
  - 502/503: Nginx upstream-not-ready response (served while the upstream
    gunicorn process is still starting up behind the reverse proxy)

Any other HTTP response is treated as "service is up". Polls every 2
seconds for up to 120 seconds, then raises RuntimeError with a clear
message.

This prevents spurious failures when smoke tests are queued immediately
after a service restart.

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