The repo had no automated tests, so regressions in the email
parsing pipeline could only be caught manually. The reusable
ci-python-sam workflow already supports a run-tests input at the
pinned SHA; enable it and add a first suite covering the pure
functions pad_zip and parse_raw_email.
tests/conftest.py sets AWS_DEFAULT_REGION and dummy credentials
before any handler import because the handlers create boto3
clients at module import time, and loads each handler.py via
importlib under a unique module name since the files share a
basename. pytest.ini scopes discovery to tests/ so the manual
test_local.py script at the repo root is not collected.
