- Add pull_request trigger so the extension-test job runs as a visible
  status check; build/deploy stay gated off pull_request events.
- Cache .adxvenv on Pipfile.lock — the dominant cost is bootstrap's
  'pipenv sync --dev' (CKAN + ~19 extensions), not the image build.
- Cache the unaids React node_modules on its yarn.lock.

Both caches are safe on miss, so the first run is a clean cold signal.

- Run all 5 extension suites and aggregate, so one run reports the full
  picture instead of stopping at the first failing suite.
- Split cache restore/save so .adxvenv and node_modules are saved even
  when tests fail, making triage runs warm instead of cold.

- Add 'mock' and 'pyfakefs' to dev-packages: ckanext-validation,
  -scheming and -emailasusername import the standalone 'mock' package
  (and pyfakefs in validation), which weren't installed, causing
  pytest collection errors. Dev-only — prod uses 'pipenv sync' without
  --dev, so these don't ship.
- run_tests: blank CKAN_SMTP_SERVER for test runs so suites never reach
  the dev stack's smtp4dev. Fixes ckanext-unaids'
  test_send_dataset_transfer_emails_errors, which asserts mail sending
  fails when no server is configured.

The validation/unaids extensions pin frictionless[ckan]==5.13.1 and
pyfakefs==4.6.* in their own requirements and pass their own CI against
those. Our merged Pipfile had frictionless>=5.0.0,<6.0.0 (drifted to a
newer 5.x that dropped Resource.__create__) and pyfakefs=* (6.x dropped
the CreateFile API), so ckanext-validation's test suite failed in our
stack only. Aligning the pins fixes it without touching the submodule;
frictionless is prod-facing but this matches the version the extensions
are built and tested against.