When the stream consumer's first edit_message() call fails (Signal,
Email, HomeAssistant don't support editing), it now disables editing
for the rest of the stream instead of falling back to sending a new
message every 0.3 seconds. The final response is delivered by the
normal send path since already_sent stays false.
Without this fix, enabling gateway streaming on Signal/Email/HA would
flood the chat with dozens of partial messages.