Add EMAIL_ALLOWED_USERS check in EmailAdapter._dispatch_message()
to silently discard emails from senders not in the allowlist. This
prevents the adapter from creating thread context and dispatching a
MessageEvent for unauthorized senders, which could race with the
gateway authorization check and result in SMTP replies being sent
despite the handler returning None.
Test: tests/gateway/test_email.py::TestDispatchMessage::test_non_allowlisted_sender_dropped
Test: tests/gateway/test_email.py::TestDispatchMessage::test_allowlisted_sender_proceeds
Test: tests/gateway/test_email.py::TestDispatchMessage::test_empty_allowlist_allows_all
fd9c32c0f2