Signal-cli sends dataMessage wrappers for profile key updates and other
metadata events that have no actual text content. These were reaching the
gateway as msg='' and triggering full agent turns for nothing.
Add early return in _handle_envelope() when both message field is empty/
missing/whitespace AND there are no attachments. Messages with media
attachments but no text still flow through.
- 12 lines added to gateway/platforms/signal.py
- 5 new tests in TestSignalContentlessEnvelope class
f61695ee73