Previously the SSE loop and the empty-phone-number short-circuit in start()
had zero coverage. Both now have tests: a fake httpx stream feeds canned
SSE lines, exercising the valid-frame, invalid-JSON, non-200, and
no-http-client paths; start() with an empty phone number is asserted to
return without entering the HTTP loop.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>