- previous code used `.then` on a non-promise, which doesn't work on
  ruby 2.7
- also, early-return case failed to consider resend events, fixed that

- some extra generators for domain-specific data like NANPA-compliant
  phone numbers, Bandwidth media URLs, and JIDs (could be better, but
  nobody's perfect)
- Rake test tasks split into `unit` and `property` (existing `test`
  target runs both and is still the default)
- `property` target picks a random seed, then uses that seed to set
  `--seed=` (for minitest) and `srand` (for rantly). downside: you can
  only seed a whole test, you can rerun just one property with the same
  generated input. sad.
- `rake property` will, by default, collect failing seeds into
  `test/stubs`

turns out you need to set it in both test_helper and in em
- sgx_bwmsgsv2 references ARGV[0], so must set it before
requiring
- but each  test runs in its own fiber, and fibers don't share ARGV

fails, at least under ruby 3, without this

When text is empty/nil and there is no media, return 400 instead of
falling through to message construction with nil text.

When jparams['to'] is an empty array, return 400 instead of
falling through to message construction.

When the webhook references a phone number with no Redis JID mapping,
the handler returns 403 and writes no XMPP stanza.