Unverified devices (lacking cross-signing) caused OlmUnverifiedDeviceError
in _send_text(), silently breaking all message delivery. Now on_sync()
blacklists non-cross-signed devices instead of skipping them, and
_send_text() catches E2EE errors gracefully.
Adds 12 unit tests for device trust policy and send error handling.
CI test job now gates deployment in deploy.yml.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
