SaaS tidying (#6665)

# Description of Changes
* Remove complex port selection logic from `engine.yml`. It's
inconsistent with the frontend & backend task files, and caused issues
with Docker, which have been worked around but would be simpler to just
get rid of the problem altogether
* Fix Ruff formatting of Python script
* Remove payg tests which are failing and have drifted too far from the
implementation to save directly

testing/cucumber/behave.ini

@@ -5,12 +5,5 @@
 #
 #   python -m behave features/enterprise
 #
-# PAYG shadow-mode features live in features/payg/. They need the saas
-# profile + Postgres + seeded test team (testing/compose/docker-compose-saas.yml).
-# The default run excludes them; the saas-cucumber CI job invokes
-# behave with `features/payg` explicitly. The PAYG harness in
-# testing/test-payg.sh swaps a temp behave.ini that drops the payg
-# exclusion + tags @manual to skip restart-hook scenarios.
-#
-exclude_re = features/(enterprise|payg)
+exclude_re = features/enterprise
 tags = ~@manual